1 Environmental GIS Prepared by Dr. Zhi Wang, CSUF – EES Department Lab 18c: Spatial Analysis III: Clip a raster file using a Polygon Shapefile These instructions enable you to “clip” a raster layer in ArcMap 9.x using Spatial Analyst. Basically, you need a sample raster (grid) file and a mask polygon shapefile. Download Lab18c.zip to your folder. Unzip it using WinZIP or Windows XP programs. 1. CREATING A MASK (polygon with correct projections - shapefile) 1.1. Drawing a Mask Polygon Shapefile and load into ArcMap: A) Start ArcCatalog. B) Right-click your project folder and select New > Shapefile. C) Name your shapefile “Mask” and specify: • Feature Type = Polygon • Spatial Reference is Optional D) Start ArcMap. E) Add the raster layer (dem-grid), CA-watersheds, and the “Mask” shapefile (click OK) into ArcMap. Double click on legend of “CA-watersheds” and choose “hollow” (red outline). Right-click on “CA-watersheds”, choose “Label features”. Double-click on “dem-grid”, choose a color ramp. Save the project as “Lab18” (Lab16 was used before the year 2011).
Transcript
1
Environmental GIS Prepared by Dr. Zhi Wang, CSUF – EES Department
Lab 18c: Spatial Analysis III: Clip a raster file using a Polygon Shapefile These instructions enable you to “clip” a raster layer in ArcMap 9.x using Spatial Analyst. Basically, you need a sample raster (grid) file and a mask polygon shapefile.
Download Lab18c.zip to your folder. Unzip it using WinZIP or Windows XP programs.
1. CREATING A MASK (polygon with correct projections - shapefile)
1.1. Drawing a Mask Polygon Shapefile and load into ArcMap:
A) Start ArcCatalog.
B) Right-click your project folder and select New > Shapefile.
C) Name your shapefile “Mask” and specify:
• Feature Type = Polygon • Spatial Reference is Optional
D) Start ArcMap.
E) Add the raster layer (dem-grid), CA-watersheds, and the “Mask” shapefile (click OK) into ArcMap. Double click on legend of “CA-watersheds” and choose “hollow” (red outline). Right-click on “CA-watersheds”, choose “Label features”. Double-click on “dem-grid”, choose a color ramp. Save the project as “Lab18” (Lab16 was used before the year 2011).
2
F) Click on View > Toolbars > Editor, then click Editor > Start Editing. Click on a folder with “Mask” and “start editing”. Make sure Task = Create New Feature; Target = Mask.
G) Click on the sketch tool (pencil) and start drawing a polygon to include the upper Merced River watersheds (several). Follow the line and click many points along the frame lines. Double-click on the very last point. A highlighted polygon (Mask shapefile) is created.
H) Click on Editor > Stop editing and save your edits. Verify that the “Mask” file was saved properly by Right-clicking on “Mask” > Remove then adding it back into ArcMap.
3
2. CLIP RASTER (GRID) FILE USING A MASK POLYGON
A). In ArcMap, choose TOOLS - EXTENSIONS and enable “Spatial Analyst” and “3D Analyst”.
B). Choose TOOLS - CUSTOMIZE and make the Spatial Analyst and 3D Analyst toolbars visible.
C). Add the RASTER (dem-grid) and POLYGON.shp polygon (Mask) data, if not added yet. To “clip” a grid in ArcGIS, all you need to do is set the appropriate Analysis Options and then call on the Spatial Analyst Raster Calculator.
D). Choose SPATIAL ANALYST > OPTIONS. Under the GENERAL tab, set working directory as your project folder (Lab18 here) and the Analysis Mask to “Mask”. Under the EXTENT tab, select “Same as Layer Mask” as the Analysis Extent. Clock “OK”.
4
Other choices: Under the EXTENT tab, you can select “Same as Layer RASTER” as the Snap to Raster layer. This ensures that the output raster cells line up with the input cells - you may opt to ignore this or use a different layer, which may involve resampling. Under the CELL SIZE tab, you can also select “Same as Layer dem-grid” as the Analysis Cell Size. Click OK.
E). To Clip the raster file (dem-grid), choose “SPATIALANALYST” > RASTER CALCULATOR. Double click on the RASTER layer “dem-grid” to enter it in the expression box. Click EVALUATE.
If this did not work, you may need to restart your computer, start a new ArcMap and reload all files again! Close other programs and browsers that may affect the calculations.
F). A “Calculation” layer is added to the table of content (see below). Uncheck all other layers to see the clipped raster file.
5
G). Right click on the “Calculation” and MAKE PERMANENT by saving with a new name; e.g. “Project-Area”.
H). Click “Yes” to add the “Project-Area” to ArcMap. Delete the temporary “Calculation” layer (right-click on it > Remove). Double-click on legend of “Project-Area”, choose a color ramp.
6
I). Create a Hillshed file: SPATIAL ANALYSIS > Surface Analysis > Hillshede. Save output as Lab18/Project-shade. Click “OK”
J). A “Project-Shade” file is created and added to the map. In the table of content, drag the “Project-Shade” below “Project-Area”. Right-click on “Project-Area” > “Properties” > “Display”. Set “transparent” value as “50” %. Click “OK”.
K). A 3-D landscape map is produced. Now, remove “dem-grid” file (right-click > Remove).
7
J). You can also produce other spatial analyses (e.g. slope, contour, aspect etc.) to provide spatial data for geological and engineering assessments. See a slope map below (50% transparent).
K). Save your ArcMap, then close it.
8
3. Three-dimensional (3-D) analysis in ArcScne:
A). Start > Programs > ArcGIS > ArcScene B). Add “Project-Area” C). Go to “Tools” > Extensions > 3D Analyst. Create TIN10
D). Use “View” > Toolbars > “Tools” and “3D effects” to view TIN10 landscape in Yosemite Valley. Use Tools (Navigate, Zoom In/Out, Zoom to target, etc.). Save ArcScene file to Lab
9
4. How to clip a multi-band image:
Basically, you add each color band of the image file into ArcMap, then clip each of them using the same methods described above.
