topic Re: Merging Raster files in Water Resources Questions

Merging Raster files

AnanG — Tue, 16 May 2017 07:04:43 GMT

I want to merge many landuse raster data. Each raster file has its own different kinds of landuse type.

When I try to merge all of them (I have like 30+ raster files) the "Mosaic to New Raster" tool gives me an output with only 4 (the minimum number of unique values in one of the raster files) types of landuse file.

I also tried using "Combine" tool with (< 20 files) but it didn't work.

How can I merge all the landuse raster files and also get 7 unique values in the final output?

Re: Merging Raster files

DanPatterson_Retired — Tue, 16 May 2017 09:06:37 GMT

do they overlap in extent?

are they adjacent with no overlap?

Answers to these questions might suggest whether a reclass is necessary for some or all the files or whether a merge/mosaic in combination with a combine is necessary

Re: Merging Raster files

AnanG — Wed, 17 May 2017 00:45:33 GMT

What do you mean by "overlap in extent"? They all have the same geographic coordinate system.

Prior to merging, I previously reclassified them with landuse categories from 1 to 7.

Re: Merging Raster files

DanPatterson_Retired — Wed, 17 May 2017 01:01:35 GMT

extent.... left right, top bottom... are they all the same or are they side-by-side or are there some that overlap

Re: Merging Raster files

AnanG — Wed, 17 May 2017 01:13:50 GMT

Hi Dan,

One of the tiles seem to be overlapping. Here is the extent.

Re: Merging Raster files

DanPatterson_Retired — Wed, 17 May 2017 01:23:47 GMT

so you have to check that they are all in the same coordinate system. Your example is in decimal degrees, hence a Geographic Coordinate System. When you do a merge, or anything with multiple files, you have to decide whether you are working with the union (ie combination) of their extents... what to do with overlaps (if any), what to do with gaps (spaces between adjoining layers), the final cell size (if any differ) and a variety of other things. This is why I said that it may be a combination of steps/processes and a one-button solution may not be possible. So figure out exactly what you have and how to proceed incrementally. It is apparent from your previous attempt that something got left out during the process... you have to figure out why. There is nothing wrong with combining a couple of files that you know are adjacent without overlaps etc... then proceed to introduce other files as you go.

Re: Merging Raster files

AnanG — Wed, 17 May 2017 02:03:27 GMT

I checked my files again and all their extents are different. I also noticed a small gap between the adjoining layers.

How do I make sure there are no gaps and also make the extents uniform? Thanks, Dan! Appreciate your help!

Re: Merging Raster files

AnanG — Wed, 17 May 2017 03:11:24 GMT

Dan_Patterson‌ - Hi again!

I unified the extent of the each file by exporting the data and changing the pixel size to a uniform size (0.00054). I just merged it again but could not get the output. I checked the coordinate system too, it looks alright. What else could I be doing wrong?

Re: Merging Raster files

DanPatterson_Retired — Wed, 17 May 2017 03:35:49 GMT

This option looks like you are getting on track, it appears that the colour maps differ, and I would check the cell sizes of these to ensure that they are the same. Then examine the options in mosaic to ensure that you get everything aligned and color matched. I would do this with copies,... you can mosaic to new raster, but that tool keeps saying mosaic has more options. the mosaic dataset option is another to look at depending on what you want to do

Re: Merging Raster files

AnanG — Wed, 17 May 2017 04:50:33 GMT

How would you suggest me to match the colours? I used the "Mosaic Dataset Option" only to get a black coloured raster that looked like DEM. I need to put all the coloured landuse maps together for landuse analysis.

Re: Merging Raster files

AnanG — Wed, 17 May 2017 05:09:20 GMT

Hi again Dan

I tried the "Mosaic to raster tool" for two files with different classes and it merged. The dark blue map shows the merged file (One file with 6 values; other 7)

The problem is definitely with the gaps between the adjoining rasters. I tried to increase the pixel size for the raster which had gaps. But no matter how much I increase the pixel size, the gaps don't disappear. How do you suggest me to fix these gaps?

Sorry for the inconvenience. I really appreciate your help. Thank you.

Re: Merging Raster files

DanPatterson_Retired — Sat, 11 Dec 2021 23:15:44 GMT

from the support pages, one method is to use an averaging approach

To: Remove and replace no data values within a raster using statistical information from the surrounding data values.

For version 10.0

1. In ArcMap, navigate to ArcToolbox > Spatial Analyst Tools > Map Algebra and open the Raster Calculator.

2. In Raster Calculator, input the following syntax:

Code:
Con(IsNull("raster"), FocalStatistics("raster", NbrRectangle(5,5, "CELL"), "MEAN"), "raster")

Focal Statistics from help.... You might have to experiment a big. BUT!!! get everything together and quit playing around with the cell size to try and close the gaps...it does NOT work that way

Re: Merging Raster files

AnanG — Wed, 17 May 2017 06:42:41 GMT

Hi again,

Thank you for the help Dan!

I used the "Raster Calculator" and it gave me something like this. Something still seems wrong though (see the red box). Maybe my focal mean values are still smaller for that file. Moreover, it does not have any attribute table which would show the number of pixels for each landuse type. What do I do now? Sorry for troubling you so much. I feel like I am getting closer to the solution so I can't help but ask you

Re: Merging Raster files

DanPatterson_Retired — Wed, 17 May 2017 06:59:06 GMT

use the equivalent tools from Arctoolbox since you have better control over the analysis properties in the Environments tab for every arctoolbox tool. The raster calculator is nice... but get used to using the actual tools.

You do the focal statistic when the whole area is merged together... AND... if you do the mean you will end up with a floating point raster. Do the majority or something so that you get an integer raster to maintain your integer classes!

The whole purpose of the venture is to get rid of the gaps at the edges while maintaining the input data type. Don't lose sight of the important bits as you go along.