topic Re: Refine griding of raster image in ArcGIS Spatial Analyst Questions

Refine griding of raster image

Andrew_Desrosiers — Tue, 27 Jan 2015 18:30:43 GMT

Hello,

I have a population dataset that i have imported into ArcGIS. I have this data referenced to UTM39. I have two versions of this data: one the raw raster image and one where i converted the raster to polygon. This data's resolution is 30 arc-seconds.

I have generated a fish net with a grid spacing of 90 meters also has been projected into UTM39. The idea behind this project is to refine the coarser data into 90m cells so we can create a gridded file to run through our modeling computers with other parameters. We want to refine the population data into these 90m grids. If one 90m grid shares two pixels, we want to take the mean of those two pixels and keep the calcualted mean value in the corresponding 90 meter cell location.

I have tried using the identify tool, the Intersect tool, and the Union tool to no avail. What appears to be the issue is that when there are two pixels within one 90m cell location, a new column is generated snapping to the two pixels. This generates a non-uniform area grid. I would like to preserve the grid in its 90m size.

Attached is an example of the snapping I have been seeing...

How can I avoid the snapping and refine this data?

Thanks in advance!

Andrew

Re: Refine griding of raster image

MikeCusi — Wed, 28 Jan 2015 05:10:51 GMT

Mixing raster and vector inputs in any analysis is one sure way of creating this problem you are in right now. You should choose to do your analysis with either one or the other. Personally I would do all my analysis with rasters as the use of snap rasters avoids this kinds of problems. Is Spatial Analyst available for your use?

Re: Refine griding of raster image

Andrew_Desrosiers — Wed, 28 Jan 2015 15:41:50 GMT

Yes, I have spatial analyst available for use.

Re: Refine griding of raster image

Andrew_Desrosiers — Wed, 28 Jan 2015 22:54:22 GMT

Essentially I am tinning right? Just not sure how to tin my population data set.

Re: Refine griding of raster image

MikeCusi — Wed, 28 Jan 2015 23:46:54 GMT

Correct me if I am wrong but from my understanding what you were wanting was to "resample" your 30 arc-second raster image into a 90-meter raster image?

Re: Refine griding of raster image

Andrew_Desrosiers — Thu, 29 Jan 2015 00:15:53 GMT

Yes you are correct. I want to map the 30 arc second population data into the 90 meter grid cells

Re: Refine griding of raster image

MikeCusi — Thu, 29 Jan 2015 02:56:39 GMT

The simplest way to do this is to use the resample tool (on your "raw" raster, with pixel size 30 arc-second ~900m) using nearest neighbor so it will not create any new values to a pixel size of 90m. If you want a bit of smoothing, bilinear will be a good technique. Any particular reason why you want to average where two 30 arc-second pixels intersect with a 90 m pixel specially considering that it is population data?

Re: Refine griding of raster image

Andrew_Desrosiers — Thu, 29 Jan 2015 03:40:31 GMT

I want to average because neither of the two pixels within that 90m grid cell represents the real population. The average would help the transition between the neighborhoods of Kuwait.

Re: Refine griding of raster image

MikeCusi — Thu, 29 Jan 2015 03:59:00 GMT

I was just thinking that maybe it does not matter too much as you are for all intents actually working with a ~900m pixel to begin with regardless of whether you have resampled it to 90m. A grid this coarse is always never close to representing the real population. e.g 10 pixels to the left =50, 10 pixels to the right = 100, middle pixel where they meet will be 75?

Re: Refine griding of raster image

KishorGhatage — Thu, 29 Jan 2015 04:50:25 GMT

Use Zonal statistics tool. The fishnet output of 90 and the population raster would be the inputs and set the statistics type 'MEAN'. Here are the steps:

1. Go to Spatial analyst > zonal > zonal statistics

2. Zone data = fishnet

3. Zone field = any ID field

4. Value raster = population raster

5. Output raster = set output location and name

6. Statistics Type = 'Mean'

7. Environment > Processing extent > Snap raster > population raster

8. Environment > Raster analysis > Cell size > As specified Below = 90

9. Environment > Raster analysis > Mask = Population raster

10. Click Ok > Click Ok

If you would like to have output in the form of table, then use Zonal Statistics as Table.

Hope this is helpful

Kishor

Re: Refine griding of raster image

Andrew_Desrosiers — Thu, 29 Jan 2015 16:12:27 GMT

Would this map the population data directly to those 90m grid cells generated from the fish net? What I am looking for is

_________ [][][][][][][][][][]

| | [][][][][][][][][][] (10x10 - 90m)

| 900m | ---------> [][][][][][][][][][]

|________| [][][][][][][][][][].......

I am relatively new to arcGIS, and I was throw into a project and expected to figure this out, so I am sorry if my terminology and explanations are misleading (if they are). I am extremely interested in this software and learning it, just will take some time. Assume that level knowledge when explaining stuff to me too please, I am still very new at this! I have two degrees in meteorology and mathematics, so I understand one-to-one mapping, and one-to-many mapping in matrix form manipulations, however, do not know how to do it here.

Kishor I tried your method, not sure if worked or not. I see my grided 90m cells, however, it does not appear to have mapped correctly. I may be doing something wrong here, so I apologize if it is me, however, I did the procedure outlined.

Mike, I have a feeling re-sampling may be needed as well to avoid the resolution issues you were talking about. I am unsure how to fully complete that procedure. If there is an article or you can provide a procedure for me, I would greatly appreciate it!

Both you you, thank you so much for you help thus far. I am not only learning the thought process here, but learning new tools and how to attempt the same project from different angles!

Thank you so much for everyone's help so far.

Andrew

Re: Refine griding of raster image

Andrew_Desrosiers — Thu, 29 Jan 2015 18:59:53 GMT

I figured out that the resampling method uses the rasters lower left hand corner as the origin. My fishnet has to be the lower left hand origin because that is what the other data-sets that I will be doing similar resampling on start.

Re: Refine griding of raster image

MikeCusi — Thu, 29 Jan 2015 22:39:34 GMT

Resampling looks like the best option to me. You can then make sure that all your other datasets use the resampled image as your "snap" raster for all your analysis. Use the "Search| Tools" function within ArcMap to look for the tools you need.

Re: Refine griding of raster image

Andrew_Desrosiers — Fri, 30 Jan 2015 15:39:13 GMT

In order to do this, i believe I am going to have to convert my 90m-celled fishnet to a polygon so that I can snap on that. Am I right in assuming I need to convert the fishnet to some kind of polygon or raster image for snapping?