topic Re: Selecting a polygon based on raster values? in ArcGIS Online Questions

Selecting a polygon based on raster values?

LauraWiggins — Sun, 27 Nov 2016 14:21:42 GMT

Hello,

I'm currently locating a site for a new air quality monitor and have a set of raster 1X1km cell raster data which I made from a .csv file. This raster holds the mean amount of NO2 in each cell as a value.

I'm looking to put this monitor in an urban park to monitor urban background pollution and have these parks as polygons, and I also want to put them in areas where concentrations of NO2 are fairly high to keep an eye on them.

My dilemma is that I can't select the polygons which are within the high NO2 raster cells, and because I made the raster myself from point data, I can't convert it into square polygons with the value to select by location.

(It's all a bit complicated but I've tried to be as clear as I can!)

Does anyone know how I can select the polygons which have a high or low NO2 value from the raster? I've included an image to try and make it a bit clearer!

Thanks in advance,

Laura

Re: Selecting a polygon based on raster values?

RebeccaStrauch__GISP — Sun, 27 Nov 2016 20:02:09 GMT

If I'm understanding you question correctly, you are wanting to take the raster, convert it to polygons, then Dissolve the polygons based on the value to get the area with the highest values. If that is correct, take a look at

Raster to Polygon—Help | ArcGIS for Desktop And Generalizing polygon coverage data—Help | ArcGIS for Desktop

Re: Selecting a polygon based on raster values?

LauraWiggins — Sun, 27 Nov 2016 20:06:15 GMT

Thanks for your reply!

Unfortunately because I created the raster in the first place from point data, it won't convert to a polygon. I've tried it a few different ways and it won't budge. Any thoughts on why, I can't seem to figure that out either?

cheers!

Re: Selecting a polygon based on raster values?

RebeccaStrauch__GISP — Sun, 27 Nov 2016 21:17:03 GMT

My guess is it isn't an integer raster (as required, based on the first help link above )

take ke a look at Converting Floating Point Raster to Polygon which has a couple suggestions for changing the raster to an integer version....make sure you create a new raster and leave your original intact in case it doesn't work. I was going to suggest the first option, that is creating a reclassified raster from the value ranges you are looking to grab, one you hare that created, you should be able to proceed with the process I suggested.

An overview of the Reclass tools—Help | ArcGIS for Desktop

This assumes you have a Spatial Analyst license available.

hope this helps.

Re: Selecting a polygon based on raster values?

LauraWiggins — Sun, 27 Nov 2016 21:20:31 GMT

Thank you! I will give this a go!

Re: Selecting a polygon based on raster values?

RebeccaStrauch__GISP — Sun, 27 Nov 2016 21:24:16 GMT

I just noticed you posted this in ArcGIS Online....just to let you know, my info was for Desktop, so hopefully you have that. If not, you will need others to jump in. If you can do this in Desktop, you may want to move this to a different group. Hope I'm not leading you in the wrong direction.