topic Compare cell values of different raster layers in ArcGIS Spatial Analyst Questions

Compare cell values of different raster layers

BernadetteQuinn — Fri, 26 Jul 2013 19:04:54 GMT

I want to look at the impact of varying weight values in a weighted overlay analysis (it???s part of a study on the impact of uncertainty in weights on the resulting maps when using weighted overlay analysis to find suitable sites for development). I have 13 different sets of weights which sum to 100 and 13 maps, each prepared with a different set of the 13 weights. The first map using the values 65 + 28 + 7 is my base map and all other maps are a variation of the base map values. For example one map uses the values 59 + 33 + 8, another uses 69 + 24 + 7, another uses 71 + 22 + 7 and so on.
My resulting maps have cells with values between 1 and 9 with cells of value 9 being the most suitable locations for development.
For each of my maps I want to compare the total number of cells with a value of 9 against that of my base map - so I want to
compare the base map against each of the 12 other maps and look at whether and to what extent the amount and location of cells
with value 9 changes. (I want to find out to what extent uncertainty in the input weights influences the output of the weighted
overlay analysis).
Any suggestions of a suitable method to carry out this analysis would be appreciated. I have been using raster calculator to subtract each of the variation maps from the base map but I want to focus just on difference in the most suitable cells (i.e. those with a value of 9) and the raster calculator doesn???t seem to be a very efficient way of doing this.

Many thanks

Ber

Re: Compare cell values of different raster layers

TOMKONG — Mon, 29 Jul 2013 21:12:45 GMT

1.Extract the pixcel value with 9 for each overlayed output: raster dataset 1-13, 1 is from base map;
2.Convert the 13 raster outputs to 13 polygon feature classes;
3.Using #1 feature class as base data, and using the Analysis Tool (find the difference tool --ArcInfo license) to find the changes (compair to the base polygon) and unchanges.
4.Then #2....

Re: Compare cell values of different raster layers

BernadetteQuinn — Mon, 29 Jul 2013 21:46:07 GMT

Hi Tom,
Many thanks for your reply. I think I may have done something similar - I used raster calculator to subtract each map from my base
map to look at the difference between cell values. Do you think the method you suggest will produce different results to what I have done?
I see that you have suggested to extract the cells with a value of 9, but i'm still not sure of the difference between the two methods.
Many thanks,
Ber

Re: Compare cell values of different raster layers

TOMKONG — Mon, 29 Jul 2013 22:28:36 GMT

Hi Tom,
Many thanks for your reply. I think I may have done something similar - I used raster calculator to subtract each map from my base
map to look at the difference between cell values. Do you think the method you suggest will produce different results to what I have done?
I see that you have suggested to extract the cells with a value of 9, but i'm still not sure of the difference between the two methods.
Many thanks,
Ber

Ber,
In your first post, you said you want to focus on the cells changes that cell with value 9 (suitable for development) and to test the changes affected by different weight models.
So the "subtract" you used is only to get the difference between different model cell included all 1-9 values; but the extracting 9 values from different models is to focus on the best area changes affected by different assigning weight values.
Tom

Re: Compare cell values of different raster layers

BernadetteQuinn — Tue, 30 Jul 2013 13:23:12 GMT

Thank you Tom, I see now the benefits of your suggested method. I have extracted the "9"'s and convert to polygon but I cannot
find which tool you are suggesting to use to compare two polygonss to find the changes... (I have an ArcInfo license). I would appreciate if you can tell me the name of the tool if possible?
Many thanks
Ber

Re: Compare cell values of different raster layers

TOMKONG — Tue, 30 Jul 2013 15:13:12 GMT

Thank you Tom, I see now the benefits of your suggested method. I have extracted the "9"'s and convert to polygon but I cannot
find which tool you are suggesting to use to compare two polygonss to find the changes... (I have an ArcInfo license). I would appreciate if you can tell me the name of the tool if possible?
Many thanks
Ber

It's the Symmetrical Difference tool.