Spatial Analysis (Vector I) Reading Assignment: Bolstad Chapter 9 (pp )

Questions about the physical world Planning: Conduct Research Define Protocols Collect/Create/Edit Spatial Data Spatial Analysis Create Output Report Results Decide 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 Unit Watershed Layer Hydrologic Unit

Output 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)

Slope Streams Buffer Streams Buffer Land Use Land Use Soils 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

–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 Zoning Industrial Zoning Parcel

Sliver Polygon

Now called “X,Y Tolerance” Now called “X,Y Tolerance”

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