Learning Objectives Enable students to use basic vector analysis tools to answer questions. The tools include: buffer, union, merge, intersect, clip and summarize. Enable students to use basic vector analysis tools to answer questions. The tools include: buffer, union, merge, intersect, clip and summarize. Create maps that convey the results of spatial analysis. Create maps that convey the results of spatial analysis.
Overlay Operation Problems One common problem in GIS is combining layers from different sources such as the boundary between two piece of property or even two countries. These are typically combined together with "Merge" and then you can find "slivers" or areas of overlap between the polygons and "gaps" or areas where there spaces between the features. These sliver polygons can cause problems both with analysis and with creating maps. You can prevent these from occurring by setting an "X,Y tolerance" on your overlay operations.
Remedies Set an X,Y tolerance when you perform your overlay operation. Set an X,Y tolerance when you perform your overlay operation. X,Y tolerance = a definition for the minimum tolerated distance between vertices. 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. Forces nodes or lines to be coincident if they are within the specified X,Y tolerance. Manually go through and remove sliver polygons. Manually go through and remove sliver polygons. Dissolve Dissolve The "Eliminate" geoprocessing tool* The "Eliminate" geoprocessing tool* Merges selected polygons with neighboring polygons with the largest shared border or area. Merges selected polygons with neighboring polygons with the largest shared border or area. Also, see managing topology in the online help. Also, see managing topology in the online help.
Test Your Knowledge All of the following are remedies for sliver polygons, EXCEPT: Set an X,Y tolerance Set an X,Y tolerance Dissolve Dissolve Eliminate Eliminate Erase Erase Manually Manually
Overlay Operations Continued Intersection The Intersect tool finds the intersection of features between two or more layers. This tool is used frequently. The Intersect Tool maintains the attributes for both intersecting features.
Case Study: Distribution of a Rare Fungal Species An example of using intersection would be if we wanted to find the habitat for a rare species such as the mushroom shown below. If we know that the species requires : Klamath Mixed Conifer Forests Klamath Mixed Conifer Forests Annual precipitation > 50 in. Annual precipitation > 50 in. We could now take a layer that contained features of Klamath Mixed Conifer Forests and another that showed the annual precipitation was over 50 inches and intersect the two. The result would be the habitat for the mushroom.
Union The image below shows a "true" union as you may have learned in geometry. The Union tool in ArcMap combines the features from one or more layers into a single layer. In other words, it creates the "Union" of two layers, rather than a spatial union as shown above. The resulting features are the unique areas between the two layers. The Union tool in ArcGIS is not used much but is presented here to prevent confusion with the "Dissolve" tool described below. ArcMap's Union Tool does not perform a Union as shown above, for this you need to use "Dissolve" in the next section.
Dissolving This tool dissolves features in a layer together, which can be based on a common attribute value or all features together. This is a proper union. Dissolving is helpful when removing unneeded information, and typically should be executed prior to applying an area based selection. An example of this is to combine drainages for each river. <<< A layer showing the drainages, or watersheds in an area.
We can dissolve features together based on an attribute that has the same values for the features to be resolved together, in this case, the Hydrologic Unit (HU) name.
Here we have selected the dissolve tool and the "HUName" field in the attribute table.
Now the features have been "dissolved" together into larger features.
Clip The Clip tool "clips" one layer with all the features from another. The Clip Tool only maintains the attributes for the original layer. Clipping Trails Layer to Streams Buffer Clip is commonly used and is closely related to "Intersect". Notice that the only difference is that the attributes are only maintained for the first layer.
Erase The Erase tool uses one layer to "erase" area from another layer. Case Study: Find Legal Grazing Land (Post Ordinance). An example of "Erase" would be to find grazing land near a stream if you were required to restrict grazing to be at least 100 meters from the stream. You could buffer the stream to 100 meters and then "Erase" the stream buffer layer from the original grazing area layer.
Split The Split tool "splits" up a layer into multiple layers based on the features in the second layer. Identify The Identify Tool "identifies" the features in one layer that overlap with features in another layer.
Symmetric Difference The Symmetric Difference tool finds the area of features that are in one of two layers and not in the other layers. Update The Update tool appears to crop the features in one layer to avoid features in another layer: I believe the help is wrong. I believe the help is wrong. Not sure what this would be used for... Not sure what this would be used for...
Append and Merge Both the Append and Merge tools combine vector layers together without changing the spatial data. Append adds a vector layer into another EXISTING layer. Append adds a vector layer into another EXISTING layer. Merge combines multiple vector layers into a NEW layer. Merge combines multiple vector layers into a NEW layer. Test Your Knowledge The Intersect Tool only maintains the attributes for the original layer. False False True True