Select to view content in your preferred language

# Calculating Area of a Larger Polygon not Covered by Smaller Polygons

881
4
12-03-2013 04:50 AM
Deactivated User
Good morning,

I am trying to calculate the area of a larger polygon that is not covered by many smaller polygons. The larger polygon is a river and the smaller polygons are SAV beds. I have tried to calculate the area but for some reason, polygons that are even a bit away from the edge are disregarded and that makes calculating the area much more difficult. Is there a parameter I am missing when using intersect? Is there a different tool I should be using? Attached is an image of what I'm working on. The black is the river and the blue polygons are the SAV beds.

Thank you for your help! I've been stuck on this for a long time now.

[ATTACH=CONFIG]29525[/ATTACH]
Tags (3)
4 Replies
by
Deactivated User
Could you use the Erase tool, to eliminate the overlaps from your river and then calculate the area?
MVP Honored Contributor
Good morning,

I am trying to calculate the area of a larger polygon that is not covered by many smaller polygons. The larger polygon is a river and the smaller polygons are SAV beds. I have tried to calculate the area but for some reason, polygons that are even a bit away from the edge are disregarded and that makes calculating the area much more difficult. Is there a parameter I am missing when using intersect? Is there a different tool I should be using? Attached is an image of what I'm working on. The black is the river and the blue polygons are the SAV beds.

Thank you for your help! I've been stuck on this for a long time now.

[ATTACH=CONFIG]29525[/ATTACH]

Sounds like you need to use Union, not Intersect, for this problem.  That will keep all portions of all of the polygons.  You can then select the larger polygon FID where the FID of the smaller polygons is -1 to get portions of the black polygon where no smaller polygons are present.  (If both FID values are greater than -1 they overlap and if the black polygon FID is -1 then only the smaller polygons are present).  If you output to a feature class in a geodatabase, the area of the cut up polygons will automatically be calculated on every polygon.  (In my opinion, shapefiles should be avoided by GIS professionals for normal data processing operations for many reasons).
Honored Contributor
Paul,

If you have an advanced license I would use the symmetrical difference tool:

http://resources.arcgis.com/en/help/main/10.1/index.html#//00080000000r000000

Regards

Anthony
Deactivated User
Thank you all for your suggestions.  I think I need to back up and first clip my SAV layers using the rivers, then possibly calculate after doing a union.  But I will also explore the erase option and symmetrical difference.