4

I'm trying to count how many floods occur within a given grid cell in ArcMap and I can't figure it out. I've been reading some other posts on here but I haven't been able to find an answer.

I have flood records for several different years in a city. I want to count how many floods occur where. I made a fishnet grid and overlayed that over my city and points, but now I don't know how to get it to count how many points fall within each grid. Does anyone know what tool can do that?

enter image description here

  • 1
    Is the grid a polygon? There is a really easy way to do this but the grid must be a polygon.\ – Michael Stimson May 4 '16 at 22:10
  • I made the grid using the fishnet tool, so I don't think it is a polygon, but I'm new to GIS so I don't know. Is there a way to make a polygon grid? – Rachel May 4 '16 at 22:15
  • 3
    The fishnet tool has an option to create a polygon output. – Fezter May 4 '16 at 22:16
  • Yes, definitely, as @Fezter said just re-make it. If you look into your tool history resources.arcgis.com/en/help/main/10.1/index.html#/… you could re-run the tool with the same parameters just check the box that says polygons. – Michael Stimson May 4 '16 at 22:21
  • Thank you everyone for your help! I really appreciate it. I got it to work thanks to all of your help – Rachel May 4 '16 at 22:47
7

Spatial Join your points to your polygons, use INTERSECT or WITHIN, no need to keep all the attributes just the OID of the polygon is what's needed on the joined points.

Using summary statistics you can count the points.. use a summary field of FID or OBJECTID depending on what sort of data you have (shape or GDB), summary type of count and a case field of polygon id (FID_1 is the most likely candidate) this will count the IDs of the rows for each unique polygon ID.

Join your Grid polygons to the summary table and you're there.

  • 2
    Did you mean 'no need to summary statistics' @Aaron? yes, you're right, the join_count is added however this workflow shows a powerful workflow for min, max, mean, sum etc. not just answering the question but preempting the next step. This is why I do it this way. – Michael Stimson May 4 '16 at 22:32
  • Hi, I have a similar question here: gis.stackexchange.com/questions/221145/… – FaCoffee Dec 12 '16 at 17:29
5

Probably the easiest way to do this is with a Spatial Join. Provided you have a polygon layer as your grid as opposed to lines...

There are a couple of ways to perform a spatial join in ArcGIS. If you use the Spatial Join tool, you can set your target features to the polygon layer. Then your join features are the points.

You'll automatically get a new field in the output feature class called "join_count" which will tell you how many features are joined.

  • 2
    Beat me by a few seconds... There are a few ways to do this but Spatial Join is definitely the best - and works on the lowest license level. – Michael Stimson May 4 '16 at 22:19
1

I like the answers given above, but if converting each grid cell to a polygon is too time consuming then point density may be the all-raster alternative.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.