Dear ArcGIS people,

I am interested in the distribution of coastline by state and have started by intersecting coastline polygons with state boundaries.

I have then used the summary statistics tool to add up the length of the coastline per state.

What puzzles me is that when I go into the attribute table of the intersection and select all polygon length for a particular state and add them up I get a very different value for the total length (much higher) than when I do the summary statistics procedure (using SUM on length).

I thought that these two values should be very similar. Can anybody shed light on this ? How could I check the total length of the coastline per state that I calculate ?

Thanks a lot !

Juliane

What I do is very similar to the approach described here:

https://confluence.cornell.edu/display/GIS/Total+road+length+per+state+%28ArcGIS%29

I am interested in the distribution of coastline by state and have started by intersecting coastline polygons with state boundaries.

I have then used the summary statistics tool to add up the length of the coastline per state.

What puzzles me is that when I go into the attribute table of the intersection and select all polygon length for a particular state and add them up I get a very different value for the total length (much higher) than when I do the summary statistics procedure (using SUM on length).

I thought that these two values should be very similar. Can anybody shed light on this ? How could I check the total length of the coastline per state that I calculate ?

Thanks a lot !

Juliane

What I do is very similar to the approach described here:

https://confluence.cornell.edu/display/GIS/Total+road+length+per+state+%28ArcGIS%29

They should be very similar normally if not identical, assuming you are taking about lengths in only one layer when using both methods. If you are actually looking at two different layers, with one method applied to one and the other applied to another, such as before and after using intersection, the results could be very different. I'm not sure from your description exactly how you are getting the sum in the first method. Are you using the Statistics option from the tableview? A picture would really help clarify what you are describing.

Also, note that when you use intersect you will fail to capture any length of lines that fall outside any of the state polygons, since those lengths will be excluded from the output. The method you linked to used the Identity tool instead, which keeps all features, unlike intersect, so the results of the two approaches would potentially be different. Bad topology matches between the coastline layer and the state layer can cause a lot of unexpected summary outputs using intersect. Unfortunately Union won't work as a replacement for the Identity tool, since it only works with two or more polygon layers.