Lesson 53: Surface analysis tools Table of Contents Topic: Visibility analysis Concepts Calculate viewshed Visibility request Visibility Tools sample extension Line of sight Example Analyzing visibility from a freeway Exercises Calculate viewshed Use the Visibility Tools sample extension Topic: Derive data from surfaces Concepts Deriving slope Deriving aspect Computing curvature Exercise Derive slope, aspect, and curvature Lesson summary Lesson self test Goals In this lesson, you will learn how to: • calculate line of sight • calculate viewshed • derive slope from surface data • derive aspect from surface data • measure curvature of a surface Topic 1: Visibility analysis How do you determine whether two proposed microwave relay towers have an unobstructed path to each other? Given a set of locations for fire lookout towers, can the entire forest be seen? How frequently can a proposed disposal site be seen from an existing highway? Solutions to these types of problems are solved with visibility analysis. Two types of visibility analysis are line of sight and viewshed. Line of sight tells you whether a given target is visible from a particular point of observation. It answers the question, "Can I see that from here?" Viewshed analysis identifies the areas on a
Transcript
Lesson 53: Surface analysis tools Table of Contents
Topic: Visibility analysis
Concepts Calculate viewshed Visibility request Visibility Tools sample extension Line of sight
Example
Analyzing visibility from a freeway Exercises
Calculate viewshed Use the Visibility Tools sample extension
• calculate line of sight • calculate viewshed • derive slope from surface data • derive aspect from surface data • measure curvature of a surface
Topic 1: Visibility analysis
How do you determine whether two proposed microwave relay towers have an unobstructed path to each other? Given a set of locations for fire lookout towers, can the entire forest be seen? How frequently can a proposed disposal site be seen from an existing highway? Solutions to these types of problems are solved with visibility analysis. Two types of visibility analysis are line of sight and viewshed.
Line of sight tells you whether a given target is visible from a particular point of observation. It answers the question, "Can I see that from here?" Viewshed analysis identifies the areas on a
surface that are visible from one or more observation points. It answers the question, "What can I see from these locations?"
Determining whether two points can see each other is the most basic function of ArcView Spatial Analyst's visibility analysis tools. Visibility analysis is performed with the Line of Sight tool availble in the Visibility Tools sample extension.
You can perform viewshed analysis with the Calculate Viewshed option on the Surface menu. The Visibility Tools sample extension also allows you to perform viewshed analysis
Calculate viewshed
Whenever you need to know which locations can be seen from one or more observer locations, you will want to perform viewshed analysis. If, on the other hand, you want to know how visible the points are from locations specified by a point or line theme you'll perform visibility analysis. Suppose you manage a forest that supports multiple uses. You would perform visibility analysis to check whether drivers along a scenic route could see areas targeted for harvesting.
The Calculate Viewshed option on the Surface menu becomes enabled when you make a point or line feature theme and a grid theme active. The point or line features represent the observation points. Different visibility conditions, such as height offsets and angle of view, can be set.
The result of a viewshed calculation is a temporary integer grid in which each cell is assigned a visibility code. Cells assigned a value of 0 are not visible from any observation point. Visible cells are assigned a value equal to the number of points from which they can be seen.
Calculate Viewshed was used to create this grid theme showing the area visible from a Forest Service observation tower located on Keller Peak in the San Bernardino mountains. [Click to enlarge]
Like any grid, a visibility grid can be symbolized to emphasize important information.
Left: The grid that results from a viewshed calculation. Surface areas that can't be seen by any observer are symbolized in red. Areas that can be seen by at least one observer are symbolized in green. Right: The visibility grid has been reclassified by the number of observers who can see each cell. Red areas can't be seen by any observers. The visible areas of the grid are ramped from light green (one observer) to dark blue (four observers). [Click to enlarge]
By default, observation points are positioned one z unit above the surface and can see to the extent of the surface in all directions with an unlimited vertical angle of view. You can modify these default parameters by adding fields to the observation theme's attribute table. If fields with the specific names listed below (not case-sensitive) are added to the observation theme table, their values will be used as described. Each of the fields is optional and can be used by itself or in combination with others.
Individual records can have unique values within a field. In other words, different visibility parameters can be assigned to individual features in the observation theme. (If the observation theme is a line theme, all vertices within a particular feature will have the same parameters.)
• SPOT: defines the height of the observation point. If a SPOT field is not present, the surface elevation for the observation points is interpolated from the input surface theme. The SPOT field is useful when all observation points are located at absolute heights.
Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: A SPOT field has been added to the observation theme and given values of 3000 meters for each point. (The highest elevation in the grid theme is 1,966 meters.) These values might represent visibility from an airplane at each of the five observation points. [Click to enlarge]
• OFFSETA: defines an offset that is added to the surface elevation of the observation points. This is useful for observation points that are located on ground-based towers or platforms.
Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: The viewshed has been calculated using an OFFSETA field with values of 30 meters for each observation point. [Click to enlarge]
• OFFSETB: defines an offset height that is added to the target surface cells. This is useful for evaluating the visibility of objects with known heights.
Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: The viewshed has been calculated using an OFFSETB field with values of 50 meters for each record (as if each cell contained a tower 50 meters high). [Click to enlarge]
• AZIMUTH1 and AZIMUTH2 set horizontal angle limits to visibility. The scan proceeds in a clockwise direction from AZIMUTH1 to AZIMUTH2. Values are given in degrees from 0 to 360, with 0 oriented to the north.
Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: AZIMUTH1 and AZIMUTH2 fields have been added to the observation points theme table. For each point, the AZIMUTH1 values are 90 and the AZIMUTH2 values are 270. Each observer therefore has a field of vision of 180 degrees, looking southward. (The observation points are shown for reference.) [Click to enlarge]
• VERT1 and VERT2 set vertical angle limits to visibility. VERT1 sets the upper limit to the scan (maximum 90 degrees) and VERT2 sets the lower limit (maximum -90 degrees). The horizontal plane (0 degrees) is defined by the z value of the observation point plus the value of OFFSETA.
Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: This grid has the same OFFSETA and OFFSETB values, but VERT1 and VERT2 fields have also been added. The VERT1 value is 90 degrees (the default) for all points. The VERT2 value, however, is 0, which eliminates all field of view below the level of the horizon. [Click to enlarge]
• RADIUS1 and RADIUS2 limit the visible distance from each observation point. Areas outside the RADIUS2 search distance are excluded from the analysis. Areas inside the RADIUS1 search distance can't be seen in the visibility grid, but may still block the visibility of cells between RADIUS1 and RADIUS2.
Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: A visibility grid created with RADIUS1 values of 1,000 meters and RADIUS2 values of 2,000 for each point. [Click to enlarge]
• By default, the RADIUS1 and RADIUS2 distances are interpreted as 3D line of sight distances. The values can be processed as 2D planimetric distances by inserting a minus sign in front of the numbers.
Determining visibility is a computer-intensive process. The smaller the grid resolution (cell size), the longer the process. During your preliminary examples, you may want to set the Analysis Properties for Cell Size larger. For a final study, you can reset the cell size small enough to meet the study's requirements
Visibility request
Use the Visibility request to analyze visual exposure and to perform viewshed analysis.
Avenue syntax:
aGrid.Visibility(anFTab, aPrj, cellObserved)
Visibility is calculated for each cell of a grid theme in relation to features found in anFTab (a feature table). If cellObserved is true, the output grid will contain fields in its value attribute table (VAT) for each observer point containing information about whether that cell can be seen by each observer. If cellObserved is false, each cell in the output grid will contain the number of observers that can see that point
Visibility Tools sample extension
Many sample extensions come with ArcView. These extensions are located in the samples subdirectory of the directory where ArcView is installed. One of these is the Visibility Tools sample extension. After loading the extension, you will notice two new tools on the view GUI:
the Line of Sight tool and the Visible Area tool .
The Line of Sight tool allows you to drag a line of interest on the view. The result is a line of sight graphic on the view. Sections of the line that are visible from the starting point are marked in green, and sections that are not visible from the start are marked in red. With this tool, you can control target and observer offset. This would be important if you needed to find out which areas are visible from a fire lookout tower that is 15 meters off the ground.
The Set Visibility Parameters dialog. [Click to enlarge]
The Visible Area tool returns a polygon feature theme of the area that is visible from the starting point of the line.
A polygon theme of visible areas from the starting point of the line in the upper portion of the view. [Click to enlarge]
You set observer and target offsets, the field of vision, and the near and far distances. If your analysis requires a grid, you can convert the visibility theme to a grid.
The Set Visibility Parameters dialog. [Click to enlarge]
Sample extensions are included to help you successfully use ArcView. Note, however, that these samples are not supported by ESRI Line of sight
Line of sight analysis determines whether a given target is visible from a given observer's
point of view. A line of sight is drawn with the Line of Sight tool between an observer and a target (represented by the starting and ending points of the line). A grid theme must be active in a view for the tool to be enabled.
A line of sight is a grouped graphic. Portions of the terrain along the line of sight visible to the observer are drawn as green line segments; invisible portions are red line segments.
A line of sight was drawn from the lower left to the upper right over the mountain. Green line segments indicate visible terrain along the line graphic. Red line segments indicate terrain that is not visible. [Click to enlarge]
The visibility analysis assumes that a straight line is drawn between the observer and the target. It follows that a target may be visible even though the line segment leading to it is red (if, for instance, the target has a sufficient height offset). It also follows that when the view of a target is obstructed, the obstruction point will lie on a visible segment of the line of sight. These points are more easily appreciated from profile graphs.
Line of sight visibility profile for the line drawn across Mt. Ranier. You can see the profile of the mountain. Green line segments represent visible terrain. Red line segments represent hidden terrain. [Click to enlarge]
Offset heights for both the observer and the target can be set in the Set Visibility Parameters dialog. This dialog opens when the Line of Sight tool is selected. The offsets remain in effect for all lines drawn with the tool until another tool is made active. (Clicking on the tool at any time opens the dialog and allows you to change the offsets.)
The Set Visibility Parameters dialog. [Click to enlarge]
The offset heights are in the map units of the active surface theme
Example Analyzing visibility from a freeway
Bert is a GIS analyst for a state transportation department. A freeway beautification project, to consist of a steel and concrete sculpture and a waterfall, has recently been approved. It doesn't sound that beautiful to Bert, but his job is simply to evaluate the suitability of four potential sites along a particular stretch of freeway.
The themes drawn in the view are an elevation grid theme (Terrain), a
streets theme, a freeway theme, and a point theme of potential sites for the beautification project.
The sites rank equally well in all respects except visibility, the last important factor to be analyzed. The site that will be selected is the one that can be seen from the most number of locations on a seven-mile section of the freeway. To solve the problem, Bert does a viewshed calculation.
As his surface theme, Bert uses a grid theme of the local terrain. The Freeway line theme could be used as his observation theme, but the line has so many vertices that the analysis would be extremely slow. Instead, Bert creates a new theme of observation points spaced along the freeway at quarter-mile intervals.
By default, ArcView Spatial Analyst assumes that observers are located one z unit above ground level and that targets are located exactly at ground level. (The z units for this data are feet.) Bert modifies the defaults by adding fields to the observation theme table.
He adds an OFFSETA field and assigns a value of 3 to each observation point. This will locate the observers three feet above ground level, about the height of a driver. He adds an OFFSETB field and gives a value of 50 to each observation point. This means that each observer is assumed to be looking at an object 50 feet high (the height of the proposed sculpture). Other parameters, such as the observers' horizontal and vertical angles of view, are left to their default unrestricted values.
Top: A zoomed-in view of a section of freeway. Each red dot is an observation point. The two blue squares are proposed sites. Bottom: The observation theme table. The OFFSETA field sets a height above ground level for each observer. The OFFSETB field sets a height for the target.
Bert makes the Observation points and Terrain themes active, then chooses Calculate Viewshed from the Surface menu. A temporary integer grid is created and added to the view.
The grid displays visibility for the entire surface from the freeway. Green represents visibility and red non-visibility. In this analysis, if a cell is green, it means that a 50-foot high object located in that cell can be seen from at least one of the 29 observation points along the freeway. If a cell is red, a 50-foot high object located there cannot be seen from any of the observation points.
The grid theme table correlates the number of cells (Count) to the number of observers (Value) who can see them.
Left: The visibility grid output from the viewshed calculation. A 50-foot tall object located in any green cell can be seen from at least one observation point. In red areas, the object would not be visible from any observation point. Right: The visibility grid theme table. 2100 cells are seen by zero observers, 352 cells are seen by one observer, 405 cells by two observers, and so on. In this particular analysis, the maximum value is 26. There are 11 grid cells (out of a total of 16,124) that can be seen by 26 of the 29 observers.
All four potential sites are located in green areas, which means they are visible from at least one point on the freeway. But what Bert wants to know is which one can be seen from the most number of points.
To find out, he makes the visibility grid theme active and clicks the Identify tool. Then he zooms in on the proposed sites and clicks on them.
The Identify Results windows for each of the four potential sites. The Value field tells you how many observers can see the identified location. The first site can be seen from 15 of the 29 freeway observation points, the second (moving from west to east) from 13, the third from 21, and the fourth from 9. [Click to enlarge]
The best of the potential sites can be seen from 21 of the 29 observation points.
Of the four potential sites, the highlighted one can be seen from the most freeway locations
Exercise Calculate viewshed In this exercise, you will use ArcView Spatial Analyst's Calculate Viewshed function to choose between five alternative microwave relay station sites. The best site will be that which has line of sight visibility to the greatest portion of the study area. The five candidate sites are all located on peaks in the mountains to the north.
If you have not downloaded the exercise data for this module, you should download the data now.
Step 1 Start ArcView
Start ArcView and load the Spatial Analyst extension.
Note: If you are running ArcView GIS 3.1, you see a Welcome to ArcView GIS dialog. Click Cancel to close this dialog.
If ArcView is already running, close any open projects.
Step 2
Open the project
From the File menu, choose Open Project. Navigate to the surfalsa\lesson3 directory and open the project l3_ex01.apr.
Note: If you are running ArcView GIS 3.1, you see an Update l3_ex01.apr message box. Click No to dismiss this box.
When the project opens, you see a Microwave Relay Visibility Analysis view containing a grid theme of Elevation and a point theme representing the five proposed relay tower sites.
Step 3
Examine point theme attributes
The five relay tower sites are represented in the Towercov.shp point coverage.
Make the Towercov.shp theme active and click the Open Theme Table button
. Examine the attribute fields.
There is one record for each of the proposed tower sites. OffsetA is the height added to the elevation of the cell that each tower occupies, and modifies the visibility analysis.
Keep the theme table open for use in the next steps.
Step 4 Calculate viewshed for the first proposed tower site
Calculate Viewshed is useful in finding the observation point that can see the most or least number of cells. You will now calculate viewshed for each of the proposed towers.
Click the Select Record tool and select the first record in the Attributes of Towercov.shp theme table.
Make the Microwave Relay Visibility Analysis view active. Make both the Towercov.shp and Elevation themes active.
From the Surface menu, choose Calculate Viewshed.
Visibility analysis is often very time-consuming, so be patient while the new grid is being calculated.
The result is a new theme called Visibility of Towercov.shp. Rename the Visibility of Towercov.shp grid to Tower1. Turn on Tower1.
Cells given a code of 0 (red) are not visible. Those given a value of 1 (green) are visible.
Next, you'll repeat this procedure for the other four proposed sites.
Step 5
Calculate viewshed for the second proposed tower site
Calculate the viewshed for the second tower following the same procedure you used in Step 4. This time, select the second record in the Attributes of Towercov.shp theme table.
Rename the Visibility of Towercov.shp grid to Tower2. Turn on Tower2 to view the results.
Step 6
Calculate viewshed for the third proposed tower site
Calculate the viewshed for the third tower following the same procedure you used in Step 4. This time, select the third record in the Attributes of Towercov.shp theme table.
Rename the Visibility of Towercov.shp grid to Tower3. Turn on Tower3 to view the results.
Step 7
Calculate viewshed for the fourth proposed tower site
Calculate the viewshed for the second tower following the same procedure you used in Step 4. This time, select the fourth record in the Attributes of Towercov.shp theme table.
Rename the Visibility of Towercov.shp grid to Tower4. Turn on Tower4 to view the results.
Step 8
Calculate viewshed for the fifth proposed tower site
Calculate the viewshed for the fifth tower following the same procedure you used in Step 4. This time, select the fifth record in the Attributes of Towercov.shp theme table.
Rename the Visibility of Towercov.shp grid to Tower5. Turn on Tower5 to view the results.
Step 9
Choose the final microwave tower site
After viewing each of the new visibility themes, it seems that either Tower 2 or Tower 5 can see the greatest portion of the study area, but it is hard to be sure which one.
You can examine the theme table for each grid to determine the count for the visible cells. Remember, these cells have a value of 1.
Make the Tower2 theme active. Click the Open Theme Table button and note the count for cells having a value of 1. Next, make the Tower5 theme active and open its theme table. Note the count for cells having a value of 1.
Tower 5 is visible from 32,311 cells, and Tower 2 is visible from 30,824. Thus, Tower 5 has line of site visibility to more of the study area than Tower 2 and would be your choice for the microwave relay tower.
Step 10
Close the project
If you want to do the Challenge, go there now. Otherwise, close the project without saving any changes.
You have completed this exercise
Use the Visibility Tools sample extension In this exercise, you will perform some basic visibility analyses using the Visibility Tools sample extension.
If you have not downloaded the exercise data for this module, you should download the data now.
Step 1 Start ArcView
Start ArcView and load the Spatial Analyst extension.
Note: If you are running ArcView GIS 3.1, you see a Welcome to ArcView GIS dialog. Click Cancel to close this dialog.
If ArcView is already running, close any open projects.
Step 2 Open the project
From the File menu, choose Open Project. Navigate to the surfalsa\lesson3 directory and open the project l3_ex02.apr.
Note: If you are running ArcView GIS 3.1, you see an Update l3_ex02.apr message box. Click No to dismiss this box. When the project opens, you see Mt. St. Helens Visibility Analysis view containing an Elevation grid of Mt. St. Helens.
Step 3
Load the Visibility Tools sample extension
This exercise requires the Visibility Tools sample extension. You'll load it now.
To load the Visibility Tools sample extension in ArcView, make the Project window active, then choose Extensions from the File menu. Check the box next to Visibility Tools (sample) and click OK.
If Visibility Tools (sample) is not listed in the extensions dialog, you will need to copy the vistools.avx file from ArcView's samples\ext directory to ArcView's ext32 directory. These directories are located in your ArcView installation directory. A typical location (the default) for the installation directory is c:\esri\av_gis30\arcview.
If you cannot find this path, try searching for the file arcview.exe to find the correct path.
Once you've done this, the sample extension will be listed in the Extensions dialog the next time you open it.
If you don't want this sample extension to appear in the Extensions dialog permanently, you may load it temporarily by running a simple script. It will be available only for your current ArcView session.
To run this sample script, in ArcView follow these steps:
1. In ArcView's Project window, click the Scripts icon, then click New to create a new script.
2. Type the following line into the script window:
Once you've done this, the sample extensions will be listed in the Extensions dialog the next time you open it.
Step 4
Create a line of sight visibility profile
Make the Hafter grid theme active. Click the Line of Sight tool .
In the Available Grids dialog, choose Hafter as the grid to use for the line of sight analysis and click OK.
Click OK to accept the default visibility parameters. Now click and drag a line in the view. A Line of Sight Visibility Profile is created and the line of sight is shown graphically on the view. Green sections are visible along the line of sight and red sections are not visible.
Close the Line of Sight Visibility Profile. From the Edit menu, choose Select All Graphics. Press the Delete key.
In the next step, you'll create a visible area theme.
Step 5
Create a visible area theme
A visible area theme is helpful if you were interested in finding the areas that can be seen from any point that you choose in a grid.
Make the Mt. St. Helens Visibility Analysis view active. Click the Visibility tool .
In the Available Grids dialog, choose Hafter as the grid to use in the visible area analysis and click OK. Click OK to accept the default visibility parameters.
Click and drag a line in the view.
A visibility theme is created and added to your view. This is a polygon theme that depicts visible areas in a dotted pattern. The line and field of vision are graphics, so if you needed to create a grid theme from your output, you would simply choose Convert to Grid from the Theme menu.
If you like, try other vantage points and parameters to see how they affect visibility.
Step 6
Close the project
Close the project without saving any changes.
You have completed this exercise
Topic 2: Derive data from surfaces
How do you determine the location of steep slopes to help predict avalanche risk? How do you determine north and south facing mountain slopes to help predict seasonal snow melt? Where are areas on a surface likely to hold water in the event of a major flood? The answers to these questions can be found by deriving grids of aspect, slope, and curvature from surface data.
Slope identifies the maximum rate of change in values between each cell and its neighbors. Slope values are expressed in degrees from 0 to 90. You can use Avenue to express slope values as a percentage.
Aspect identifies the down slope direction of the maximum rate of change in value from each cell to its neighbors. It is the direction that the slope is facing. The values of the aspect grid express compass direction in degrees from 0 to 360. Cells which are flat, and thus have no aspect, are assigned a value of -1.
Curvature is a measure of a shape for surface. A positive curvature indicates that the surface is upwardly convex at that cell. A negative curvature indicates that the surface is upwardly concave at that cell. A value of zero indicates that the surface is flat
Deriving slope
Slope is calculated as the maximum rate of change between a cell and its neighbors. The output slope grid contains values representing degree of slope. A cell value of 15 means that the cell has a 15 degree slope.
Slope can be measured as degrees or percent of slope (this requires Avenue). Consider an angle of 45 degrees. The percentage of slope for that same angle would be measured with rise/run * 100 (1/1 * 100 = 100%). Measures of slope in degrees can approach 90 degrees and measures of slope in percent can approach infinity.
Slope may be calculated in either degrees or percent of slope.
Possible application areas include erosion analysis and construction siting.
In the example below, cells with the lowest slope have the lightest red shading. This means the lighter areas are flattest. Areas of high slope are darker red.
Slope theme of Mt. Ranier. Areas with darker shading have a steeper slope than those with lighter shading. [Click to enlarge]
Slope can be calculated in ArcView Spatial Analyst by using either the Derive Slope menu option on the Surface menu or by using the Slope Avenue request.
Avenue syntax:
aGrid.Slope( zFactor, percentRise )
zFactor is a number used for exaggeration or converting units. PercentRise is either true or false.
Example:
[Elevation].Slope(Nil, False )
When using the Avenue Slope request, a zFactor can be specified and there is an optional parameter to calculate slope in degrees or percent slope. The zFactor is useful if the measurement of the elevation is not in the same units as the measurement of distance in the grid. For example, the grid theme may be in State Plane feet and the elevation data may be in meters. The zFactor can be used to convert from meters to feet. PercentRise can be true (for percent of slope) or false (for degrees of slope).
If a cell from the active theme is No Data, the corresponding output slope cell will be assigned No Data
Deriving aspect
Aspect is the downslope direction of a cell to its neighbors. Aspect can be used to identify the orientation or direction of a hillside. The cell values in an aspect grid are compass directions ranging from 0 to 360. North is 0 and in a clockwise direction, 90 is east, 180 south, and 270 west.
Compass directions.
A cell with an aspect value of 90 is angled or facing east. If you walked down that hill, you would be walking east. This cell would get lots of sun in the morning as the sun rises and less sun at dusk.
In the example below, areas facing a southerly (south, southeast, southwest) direction are identified in green and light blue shades. Areas sloped toward the sun (southerly) may be better for growing crops or good for saving on heating bills (in countries north of the equator).
Aspect theme of Mt. Ranier. [Click to enlarge]
Input grid cells that have 0 slope (flat areas) are assigned an aspect value of -1.
Aspect can be calculated in ArcView Spatial Analyst either by using the Derive Aspect option on the Surface menu or by using the Avenue Aspect request.
Avenue syntax:
aGrid.Aspect
Example:
[Elevation].Aspect
If any neighborhood cells surrounding the processing cell are No Data, they are assigned the value of the processing cell and the aspect is then computed
Computing curvature
The Curvature request produces five measures of the shape of the surface at every cell. It works by fitting a fourth-order polynomial to a 3 x 3 cell neighborhood centered on each input cell.
The curvature grid is the primary output of the Curvature request. It is a measure of the overall curvature for each cell and is essentially the second derivative of the surface, or the slope of the slope. Negatives values are upwardly concave (a bowl). Positive values are upwardly convex (a hump). Values of zero are flat.
Curvature.
The profile curvature is a measure of the rate of change of curvature in the direction of greatest slope through a cell. Negative values can be interpreted as indicating decelerating flow through the cell. Positive values indicate accelerating flow. Values of zero indicate no change (not necessarily flat).
Profile curvature.
The planform curvature is a measure of the rate of change of curvature perpendicular to the direction of greatest slope. Negative values can be interpreted as indicating convergent flow into the cell. Positive values indicate divergent flow out of the cell. Values of zero indicate no change (again, not necessarily flat).
Planform curvature.
The units of the output curvature, profile, and planform grids are expressed as one over 100 z units, or 1 / 100 z units, and will generally fall in the range of –4 to +4. Values ranging between –4 and +4 may be considered mountainous (extreme relief), between –0.5 and 0.5 may be considered hilly (moderate relief), and values of 0 are flat.
The curvature slope grid measures the maximum rate of change in z (elevation) through the cell. Values are expressed in degrees and range from 0 (flat) to 90 (vertical). The curvature slope values may differ from values computed with the Slope request because of the different methods used to compute them.
The curvature aspect grid measures the direction of the maximum rate of change in z through the cell. Values are azimuthal degrees from 0 to 359.99, starting with 0 at north. Again, the curvature aspect values may differ from values computed with the Aspect request.
From an applied viewpoint, the output of the Curvature request can be used to describe the physical characteristics of a drainage basin in an effort to understand erosion and runoff processes. The slope (slopeFN) affects the overall rate of movement down slope. Aspect (aspectFN) defines the direction of flow. The profile curvature (proCurvFN) affects the acceleration and deceleration of flow, and therefore influences erosion and deposition. The planform curvature (planCurvFN) influences convergence and divergence of flow.
See the topic on the Curvature request in the online help for details on the algorithm of the Curvature request
Exercise
Derive slope, aspect, and curvature In this exercise, you will use ArcView Spatial Analyst to derive slope, aspect, and curvature to help find suitable locations for an orange grove. The criteria you will use are fictional but they illustrate the use of the tools. Assume that orange trees like:
• Low to moderate elevations: < 2000 feet (oranges like warmer areas)
• Low, nonflat slopes: 2 to 12 degrees (a slight slope helps prevent frost from settling)
• South-facing slopes: 134 to 225 degrees from north (oranges like lots of sunlight)
• Depositional soils (depositional areas are exposed to surface water longer and generally consist of well-drained sandy soils)
You will use ArcView Spatial Analyst's surface analysis tools to create grids for each of these criteria and use them in a model at the end of the exercise.
If you have not downloaded the exercise data for this module, you should download the data now.
Step 1 Start ArcView
Start ArcView and load the Spatial Analyst extension.
Note: If you are running ArcView GIS 3.1, you see a Welcome to ArcView GIS dialog. Click Cancel to close this dialog.
If ArcView is already running, close any open projects.
Step 2
Open the project
From the File menu, choose Open Project. Navigate to the surfalsa\lesson3 directory and open the project l3_ex03.apr.
Note: If you are running ArcView GIS 3.1, you see an Update l3_ex03.apr message box. Click No to dismiss this box.
When the project opens, you see an Orange Grove view containing an Elevation grid theme.
Step 3
Create a grid of suitable elevations
You'll start by creating a grid of suitable elevations using Map Query.
From the Analysis menu, choose Map Query. In the Map Query dialog, enter the
following expression:
[Elevation] <= 2000
Click Evaluate.
Rename the Map Query 1 theme to Melvgrd. Close the Map Query dialog and turn on Melvgrd.
Map query assigned a value of 1 to suitable elevations and 0 to all others.
Step 4 Derive slope
Slope computes the greatest rate of change in z over distance (the first derivative of the surface). The slope can be expressed in degrees or as a percentage of the rise over the run. Degree values can range from 0 (flat) to 90 (a vertical cliff), while percent slopes can range from 0 (flat) to 1 (rise is equal to the run) to infinity (vertical cliff). The Derive Slope option on the Surface menu measures only the degree of slope, while the Slope request can express slope in either degrees or as a percentage.
You'll create a grid of slope with values expressed in degrees.
Make the Elevation theme active, then from the Surface menu, choose Derive Slope.
Rename the Slope of Elevation theme to Slopegrd. Turn on Slopegrd and turn off all other themes.
Now examine statistics for Slopegrd. Double-click on Slopegrd to open the Legend Editor.
In the Legend Editor, click the Statistics button. The statistics give you a general idea of how steep the surface is. Things look pretty good.
The darker shaded areas are the steepest slopes. Remember that degrees can range from 0 (flat) to 90 (vertical). Are there many cliffs in the area? Even though slopes can reach 90 degrees, anything above 30 is usually considered a steep slope.
Click OK to close the Statistics window. Close the Legend Editor.
Step 5
Create a grid of suitable slopes
Now you'll use Map Query to create a grid of suitable slopes for the orange tree model. Remember, these are slopes from 2 to 12 degrees.
From the Analysis menu, choose Map Query. In the Map Query dialog, enter the following expression:
( [Slopegrd] >= 2 ) and ( [Slopegrd] <=12 )
Click Evaluate.
Rename the Map Query 1 theme to Mslpgrd. Close the Map Query dialog.
Turn on Mslpgrd. Turn off all other themes.
Map Query assigned a value of 1 to suitable slopes and 0 to the rest.
Step 6 Derive aspect
Aspect finds the direction of the greatest rate of change in z (that is, the orientation of a cell to north, south, etc). The directions are returned in decimal degrees measured from north, which has values of both 0 and 360.
Now you'll create a grid of aspect.
Make the Elevation theme active. From the Surface menu, choose Derive Aspect. Rename the Aspect of Elevation theme to Aspctgrd.
Turn on Aspctgrd. Turn off all other themes.
Get statistics for Aspectgrd.
The statistics give you a general idea of aspect over the surface.
• Minimum Value = -1.000 • Maximum Value = 359.999 • Mean = 214.712 • Standard Deviation = 87.709
Close the Statistics window and the Legend Editor.
Step 7
Create a grid of suitable aspects
In this step, you'll use Map Query to create a grid of suitable aspects for the orange tree model. These are the southerly aspects (135 to 225).
From the Analysis menu, choose Map Query. In the Map Query dialog, enter the following expression:
( [Aspectgrd] >= 135 ) and ( [Aspectgrd] <= 225 )
Click Evaluate. Rename the Map Query 1 theme to Maspgrd. Close the Map Query dialog.
Turn on Maspgrd. Turn off all other themes.
Map Query assigned a value of 1 to suitable aspects and 0 to the rest.
Step 8
Use Curvature
The Curvature request returns up to five grids that are measures of the behavior of the surface. The output curvature grid is the rate of change of the slope (slope is the first derivative of the surface, curvature is the second). The profile curvature grid is a measure of the acceleration or deceleration of flow over the surface and may be used as a measure of erosion and deposition. The planform curvature is a measure of the convergence and divergence of the flow. The function may also return grids of slope and aspect, which are computed with somewhat different methods than those used by the Slope and Aspect requests.
Use the Curvature request to create a curvature grid and a profile curvature grid of the surface. You will not use the curvature grid, but you must create one anyway as it is a required parameter of the request. You will not need the optional planform, slope, and aspect grids, so specify Nil for these parameters and they will not be created. For the profile curvature filename, specify your surfalsa\lesson3 directory and name this output grid profgrd.
From the Analysis menu, choose Map Calculator. Enter an expression similar to the following, paying special attention to the profile output filename:
Ignore the Map Calculation 1 theme when it appears as you will not use this curvature grid. Rather, load the profile curvature grid.
From the View menu, choose Add Theme. Navigate to the surfalsa\lesson3 directory and load the profgrd grid you just created (make sure the Data Source Type is set to Grid Data Source).
Close the Map Calculator dialog. Turn on Profgrd. Turn off all other themes.
Get the statistics for Profgrd. The statistics should appear as follows:
• Minimum Value = -0.398 • Maximum Value = 0.491 • Mean = 0.002 • Standard Deviation = 0.045
Negative values of profile curvature indicate depositional surfaces (water is decelerating as it flows over the surface). These are the light-colored areas of the display.
Step 9
Isolate negative, positive, and zero values
Next, you'll create a display of negative, positive, and zero only. The Con request is useful for this. Assign positive areas a value of 3, negative areas a value of 2, and areas that equal zero a value of 1.
Open the Map Calculator and enter the following expression:
When the Map Calculation theme appears, rename it to depgrd. Close the Map Calculator.
Turn on depgrd. Turn off all other themes.
There is a custom legend file provided to help you better visualize this grid. To load it, double-click on depgrd to open the Legend Editor. In the Legend Editor, click Load and navigate to the surfalsa\lesson3 directory and load the legend file deposit.avl.
Click OK to load all the legend. Click Apply to apply the legend and then close the Legend Editor.
The green areas are depositional surfaces and the red are erosional. The white area near the center is the sand and gravel quarry in the Santa Ana River wash. The bottom of the quarry is flat, so there is no change in slope.
Step 10 Create a grid of suitable depositional surfaces
Now you'll use Map Query to create a grid of depositional surfaces for the orange tree model.
From the Analysis menu, choose Map Query. In the Map Query dialog, enter the following expression:
[Profgrd] < 0
Click Evaluate.
Rename the Map Query 1 theme to Mprogrd. Close the Map Query dialog.
Turn on Mprogrd. Turn off all other themes.
Map Query assigned a value of 1 to depositional areas and 0 to the rest.
Step 11
Combine the suitability grids
Finally, you'll add the four suitability grids together. The No Data areas in each will block out all the unsuitable locations, leaving only values of 1 for the sites that meet all criteria.
From the Analysis menu, choose Map Query. In the Map Query dialog, enter the following expression:
[Melvgrd] and [Mslpgrd] and [Maspgrd] and [Mprogrd]
Click Evaluate. Rename the Map Query 1 theme to Sitegrd. Close the Map Query dialog.
Now display the final result superimposed over the original elevation grid.
Turn on Elevation and the Sitegrd themes. Turn off all other themes.
The red areas are those that meet all your surface-based criteria. Again, the criteria were made up just for this exercise.
Step 12 Close the project
Close the project without saving any changes.
You have completed this exercise. Treat yourself to an orange
Summary
In this lesson, you learned about some of ArcView Spatial Analyst's surface analysis tools.
Visibility analysis includes line of sight analysis and viewshed analysis. Line of sight analysis determines whether a given target is visible from an observation point. Viewshed analysis determines the areas that can be seen from one or more observation locations. You can create a line of sight profile graph using the Line of Sight tool. You can create a visibility grid using the Calculate Viewshed option.
Slope calculates the maximum rate of change between a cell and its neighbors and is measured either in degrees or percent of slope. Slope is calculated in ArcView Spatial Analyst by using either the Calculate Slope option on the Surface menu or by using the Slope request.
Aspect identifies the downslope direction of a cell and is measured in compass degrees ranging from 0 to 360. Aspect is calculated in ArcView Spatial Analyst by using either the Derive Aspect option on the Surface menu or by using the Aspect request.
The Curvature request calculates the curvature of a surface at each cell center, and optionally, the slope and aspect
This is the Introduction to Surface Analysis with ArcView Spatial Analyst - Lesson 3 Self test.
Please watch your time—you have 2 hours to complete this test.
Use the knowledge you have gained in Introduction to Surface Analysis with ArcView Spatial Analyst to answer the following questions. You will need to correctly answer 7 of the following questions to pass.
Netscape Users: Do not resize this browser window. This can cause the page to reload and generate new questions.
GOOD LUCK!
1. A cell with an aspect value of 0 is:
nmlkj Angled towards the north nmlkj Angled towards the south nmlkj Flat nmlkj Vertical
2. A visibility grid is a: nmlkj Temporary integer grid nmlkj Permanent integer grid nmlkj Temporary floating point grid
nmlkj Permanent floating point grid
3. The Calculate Slope option on the Surface menu returns a slope grid containing values representing percent of slope. nmlkj True nmlkj False
4. The Calculate Viewshed choice on the Surface menu is enabled only when a point or line feature theme and a grid theme are active. nmlkj True nmlkj False
5. In the Line of Sight dialog, the height offset units are in: nmlkj Meters nmlkj The map units set in the View properties nmlkj The distance units set in the View properties nmlkj The units of measure of the active surface theme
6. The downslope direction of a cell to its neighbors is its: nmlkj Aspect nmlkj Curvature nmlkj Planform nmlkj Profile
7. Two lines of sight drawn between the same points in the same direction will look identical, regardless of the observer and target height offsets. nmlkj True nmlkj False
8. When using Calculate Viewshed, the extent of the output visibility grid can be restricted to the extent of the observation theme. nmlkj True nmlkj False
9. To constrain an observer's ability to see to the extent of the surface in a viewshed calculation, the following fields should be added to the observation theme table: nmlkj AZIMUTH1 and AZIMUTH2 nmlkj RADIUS1 and RADIUS2 nmlkj VERT1 and VERT2 nmlkj OFFSETA and OFFSETB
10. What does a profile graph of a line of sight plot?
nmlkj A direct sight line between observer and target nmlkj A visibility line that follows surface contours against a direct sight line nmlkj A visibility line that follows surface contours nmlkj A visibility line that follows surface contours against the steepest path leading from the observer
