Ask A Cartographer

Density Maps

February 13 2009 | 0 comments
Categories: Map Data

When I created a density map using a series of points the output seems to be cut off on the sides. This should not be the case as the density maps should be continous should it not? If there is a point on the outside a radius should be built around it and not stop abruptly. If anyone can answer why this is that would be great.
Thanks.

Mapping Center Answer:

A density map is indeed a continuous surface. The analysis results will cover only the area that has points in it, so the analysis extent is the same as the extent of your points layer.

If the analysis results did not cover the entire area with points in it, then you may have somehow set your analysis extent to be smaller. To change it back to the extent of your points layer, in the Spatial Analyst toolbar, click Options, then click the Extent tab and set the Analysis Extent to be the Same as your points layer.

The analysis results will not extend beyond where there are points. If you want the analysis to extend beyond where you have points, so that you can see the density around the points along the edge, you can add some "dummy" points outside your original data extent, but make sure that you add them far enough away from your original data that they do not skew the analysis results. In the Density tool in ArcToolbox, this is the Radius in the Neighborhood Settings. In the Density tool in Spatial Analyst toolbar, this is the Search Radius.

Note that while this IS a mapping question of sorts, you might want to direct this type of question about Spatial Analyst tools to the Geoprocessing Forum on our Geoprocessing Resource Center. There they have a special forum for users of the Spatial Analyst Extension.

If you would like to post a comment, please login.

Contact Us | Legal | Privacy |