Spatial Analysis (Vector I)

Reading Assignment: Bolstad Chapter 9 (pp. 347-376)

Questions about the physical world Planning:

• Conduct Research• Define Protocols

Collect/Create/Edit Spatial Data

Spatial AnalysisCreate OutputReport ResultsDecide and Act

Spatial Analysis

The output from a spatial operation may be spatial (i.e. a new data layer is produced.Or the output may be aspatial (e.g., a single value, or a table)

Geoprocessing Tools Perform a Spatial Operation

– Selection (select by attribute or by location)– Conversion between data models (e.g. vector to raster)– Buffering– Dissolving– Overlay

• Clip• Intersection• Union• Erase

Spatial Operations (examples):

Buffering

• Creates polygons around existing vector features• ArcToolbox -> Analysis -> Proximity -> Buffer

Fixed Distance Buffer

• Delineates an area a fixed distance from input features (same distance).

Variable Distance Buffer• Buffers at variable distances

Specified by a value in Att. Table.

Intersection

• The overlap between two polygons

Wikipedia

Union

• The combined area of two polygons

Esri does not use this for the term “union” Wikipedia

Dissolving

• Dissolves features in a layer together – Based on a common attribute value– Or all features together

• This is a proper Union

Dissolving:• Helpful in removing unneeded information • Prior to applying an area based selection• Example: Combine drainages for each river

Hydrologic UnitHydrologic Unit

Watershed Layer

Hydrologic Unit

Output of Dissolve on Hydrologic UnitOutput of Dissolve on Hydrologic Unit

Spatial Operation: Overlay

Combination of separate data layers to derive new information (a new layer).

Both spatial and (sometimes) attribute data are combined.

(Vector Overlay -- Vineyard)

SlopeSlope

StreamsBuffer

StreamsBuffer

Land Use

Land Use

SoilsSoils

Wetlands ranked according to their vulnerability to nutrient runoff.

Overlay Operations

• “Clips” one layer with all the features from another

Clip

Clip:

Clipping Trails Layer to Streams Buffer

Identify parts of trails that are within 50 Yards of a stream

Intersect

• Finds the intersection of features between two or more layers

Intersect

Case Study: Distribution of a Rare Fungal Species

Habitat Requirements: • Klamath Mixed Conifer • Annual precipitation > 50 in.

Goal: Find areas that meet these criteria(i.e., suitable habitat)

Erase

• Uses one layer to “erase” area from another layer

Erase

Case Study: Find Legal Grazing Land (Post Ordinance).

• New ordinance: No cattle grazing within 100m of a stream.• Find total amount of land where grazing would still be

permitted.

Split

• Splits up a layer into multiple layers based on the features in the second layer

Identity

• “Identify” the features in one layer that overlap with features in another layer

Symmetric Difference

• Finds the area of features that are in one of two layers and not in the other layers

Union

• Combines the features from one or more layers into a single layer. The resulting features are the unique areas between the two layers (not a true Union)

Update

• Appears to crop the features in one layer to avoid features in another layer:– I believe the help is wrong– Not sure what this would be used for

This is the actual area of the features from layer 1 (i.e. they are no longer rectangular)

Append & Merge

• Both these tools combine vector layers together without changing the spatial data.

• Append: Adds a vector layer into another EXISTING layer

• Merge: Combines multiple vector layers into a NEW layer

A problem in vector overlay:Multiple layers representing the same thing, or boundary line.

Example:

Layer 1: Incorporated areas

Layer 2: Unincorporated areas

The boundary differs,

but represents same

boundary on earth’s surface. Sliver Polygons

• Sliver Polygons– Take up space, but are not of any interest.– A significant increase in processing times. – Erroneous analysis results.

• Remedies– Set an X,Y tolerance when you perform your overlay operation.

• X,Y tolerance = a definition for the minimum tolerated distance between vertices

• Forces nodes or lines to be coincident if they are within the specified X,Y tolerance.

– Manually go through and remove sliver polygons – Dissolve – The “Eliminate” geoprocessing tool*.

• Merges selected polygon with neighboring polys with the largest shared border or area

*Requires an Advanced license.

Residential ZoningResidential Zoning Industrial ZoningIndustrial Zoning

ParcelParcel

Sliver PolygonSliver Polygon

Now called“X,Y Tolerance”

Union operation

With a 10 Ft X,Y ToleranceWithout an X,Y Tolerance