Clipping an Ocean Polygon
When building a geographic database representing a coastal area, your census data and your digital elevation model will extend beyond the shoreline and make it difficult to make maps that look decent. It may be easy to find a representation of the shorelinne as the edge of a polygon representing the land (such as a state boundary polygon) but it is difficult to find a polygon that we can use to represent areas that should be covered by the ocean.
To download some sample data Right Click Here
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ESRI Streetmap implies the ocean as the area beyond the land. But this is not helpful when we have detail such as census data or terrain data. that extends beyond the outline of the land area. What we need is an ocean polygon that can be used to cover the innudated area of census tracts. or terrain information. |
The Basic Procedure
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The basic procedure for creating an ocean polygon given a shape that represents land is to make a new shape file and add a large rectangle to it. Then we use a function called Union to cut the new polygon using the existing land polygon. We then select the areas that are not ocean, and delete them. Voila! A useful Ocean Polygon! |
The Technical Details
There are a few techniques that you need to know in order to do this. Here are the details with citations from ArcMap On-Line Help, highlighted like this.
This procedure assumes that you have a polygon representing the land area. I suggest getting this from the esri streetmap application, as discussed in the GSD Online Manual page Beginning a GIS Database. The most detailed shoreline comes from the layer States (Local).
To download some sample data Right Click Here
- First, you need to create a new shapefile to define your clip area. Shapefile, creating. Import the coordinate system from your existing land layer. If the coordinate system of your new layer doesn't match that of the land area, the union function won't work.
- Start an Edit Session in the directory that contains your new clip shape file.
- Read the online help sections on Editing Features and add a polygon to your shape file that covers your entire study area.
- Close your editing session
- Look at the arcmap help section, Union Tool/Command (Analysis Toolbox) and union your clip polygon with the shapes from the land layer. You can find the union tool in the toolbox panel, which can be accessed by clicking the little red toolbox icon on your ArcMap toolbar. The union tool is in the Analysis Tools toolbox.
- Start editing the output from the union command.
- Select the areas that are land, and delete them. See Deleting Features. Hint: You can use the techniques coverd Selecing records in tables Selecing rcrods by attributes, and switching the selction, to efficiently select the non-land, and then the land polygons, using the attribute table of your new layer.
- Stop Editing