Overlay

Overlay geoprocessing tools for feature classes perform operations that use the geometries and attributes of two or more input feature class layers to create an output layer to meet the needs of the user. Four different overlay operations (clip, intersect, union, merge) can be performed following a similar step-by-step process:

The Layer(s): drop-down menu displays several options for selection of layers to use in each operation:

• Click on an existing Geospatial/User/Project feature class layer. Click Add to add the file to the Geometries: layers list. To remove the file, click on the X to the right of the filename. Only those layers that are polygons will work with this tool.
• A user-created Polygon can be drawn on the map by clicking on the desired option and clicking Add. Place the cursor in the desired location on the map and click to define a point and/or move the mouse and click again as needed to define the polygon of interest. Double-click to complete the shape and add the shape to the Geometries: layers list in the Union Geometries pop-up box. To remove the shape, click on the X to the right of the filename. This is where the option to draw lines and points must be removed.
• A Well-Known Boundary can also be used. With this option, the Select Well-Known Boundary pop-up box appears with a Boundary Type: drop-down selection menu.  Specific watershed hydrologic unit code (HUC) boundaries can be selected. After selecting the appropriate code by clicking on it, click Add to add it to the Geometries: list in the Union Geometries pop-up box. To remove the boundary, click on the X to the right of the filename.

Once the layer(s) are selected and appear in the Geometries: list in the pop-up box, click the operation button. A pop-up dialog box opens to confirm the success of the operation and where the new file is stored. The default filename for the new output feature class layer will appear under the Result: section of the operation pop-up box. Click on the pencil icon to add a display name to the resulting file, type in the new name, and click Save. The pop-up boxes can be closed by clicking Done/Ok.

Clip

Clip changes the geometry of the input feature layer using the geometry of the clip feature layer. The operation only preserves the attributes of the input layer.  Click on the Map Tools icon to open the toolbox and then click on Geoprocessing. Click on Feature Class and then select Overlay. Select the operation of interest from the list.

To Clip a feature class layer based on the geometry of another feature class layer, click Clip. The Clip Geometries pop-up dialog box will appear.

The available Geospatial/User/Project feature class layers to which the clipping operation will be applied are shown in the Layer(s): drop-down menu. Click on a layer to select it.

Follow the steps provided above in the introduction to Overlay features.

Intersect

New spatial information can be generated by the intersection of features. Click Intersect and the Intersect Geometries pop-up dialog box will appear.  Both the Layer(s): and Intersecting: drop-down menus display several options for selection of layers for the intersection operation.

Note: the geometry of the selected Layer: must be dimensionally higher than that of the object intersecting it. For example, polygons can be intersected by other polygons, lines or points. Lines can be intersected by lines or points, and points can be intersected by other points. 

The Intersecting: drop-down menu is used for selecting which layer or object will intersect the Layer: of interest. The output layer contains attribute information from both the Layer: and Intersecting: files.

To perform a strict intersection of the selected Intersecting: layer, click the Strict checkbox. This means that instead of finding the intersection of the union of all the given geometries, the action will find only the geometries that intersect every geometry, including the selected target.

Follow the steps provided above in the introduction to Overlay features.

Union

The Union tool will combine feature and attribute information from a set of polygon input feature class layers. The output of this process contains attributes of each of the input layers and combined attributes for areas of unique intersection between the inputs. Click Union and the Union Geometries pop-up dialog box will appear.

Follow the steps provided above in the introduction to Overlay features.

Merge

The Merge tool combines multiple input feature class layers of the same geometry type into a new merged object. The feature class layers to be merged do not need to overlap. The new feature class layer file will retain all attribute information from the merged layers, the input geometries will not be modified, no new features will be created, and the boundaries of each object merged will not physically change. Click Merge and the Merge Geometries pop-up dialog box will appear.

The Layer(s): drop-down menu displays several options for selection of layers to join using the Merge operation. Only features with the same geometries may be merged. Also, user-created polygons, lines, or points should be added to the Geometries: list after other layers are added.

Follow the steps provided above in the introduction to Overlay features.

Dissolve

The Dissolve geoprocessing tool aggregates feature class layers based on selected attributes through removal of the boundaries between adjacent polygons. The Dissolve operation does not retain all of the underlying attribute data of the original feature class layers, but creates new information for the dissolved feature.

The Dissolve tool aggregates feature class layers by combining adjacent polygons that have a common value for a specific attribute. It essentially is a tool for removing or aggregating small polygons to larger units. To use the Dissolve tool:

1. Select Dissolve from the Feature Class toolset in Geoprocessing in the GIS toolbox to open the pop-up box
2. Under the Layer to Dissolve: drop-down menu select the feature class layer of interest.
3. Once the layer of interest is selected, a threshold can be set, units of square meters (m^2), to limit the size of the area to be dissolved. Polygons with a real extent under the threshold will be processed.
4. Select candidates by attribute:
   • The Find candidates by attribute: drop-down menu lists the attribute table fields that are candidates for use in the dissolve operation. After selection of a candidate attribute field
5. Select the Critiera: using the drop-down menu below in terms of using the Same Field Value for the Dissolve or using a Non-Null Field Value
6. The Explode multipolygons checkbox separates multipart feature class polygons into independent features, with each assigned identical attribute information.
7. Clicking the Dissolve link performs the Dissolve operation. When successful, a pop-up dialog box provides statistics on the result including, how many features were removed and the total count of features. It also provides the size of the smallest feature in square meters (m^2).
   • As the Dissolve operation alters the attribute table of the selected layer, the layer of interest can be copied if desired to make a new layer by clicking the Copy link to the right of the drop-down menu and re-naming the file or using the default naming syntax. The new layer will then appear in your Geospatial/Project/User layers tab(s) and also under the Layer to Dissolve: drop-down menu list
8. The Preview link opens a pop-up dialog box informing the user of how many features would be removed and what the size of the smallest feature is in square meters (m^2). Close the Preview box by clicking Ok.

Buffer

The Buffer geoprocessing tool creates a buffer polygon of a specified distance or width around an input feature such as a point, line or polygon.

Click on the Buffer tool to open the Buffer layer Features pop-up box.

1. Under the Layer to buffer: drop-down menu, select the feature class layer of interest.
2. In the Buffer size: box enter the desired buffer width from the feature and select the desired units of meters, kilometers, feet, or miles from the drop-down menu.
3. Click the Buffer button to complete the operation. When complete, a pop-up dialog box will open displaying the name of the new feature class layer created from the operation. You can modify the name by clicking in the box where the default name is displayed.
4. Click Ok to close the pop-up dialog box and Close to close the Buffer Layer Features pop-up box.