topic Re: Image problem when mosaic to raster in Data Management Questions

Image problem when mosaic to raster

bernardfoong — Mon, 24 Sep 2012 14:57:57 GMT

Hi,

I have mosaic several DEMs to a raster but I have encountered a problem whereby the output raster produces an image where one portion has additional black layer on it. I have attached the image in this post.

Please help. I have spend a lot of time trying to solve this. As I am very new to this software, I would really appreciate if anyone could give me a step-by-step guidance with this.

Many thanks in advance.

Re: Image problem when mosaic to raster

ShaunLam — Mon, 24 Sep 2012 22:59:06 GMT

What process are you doing to mosaic ?

Maybe try creating a gdb and a raster catalog, load the catalog with your DEMs then use the raster catalog to raster dataset tool.

In ArcCatalog, right click > new > file/personal gdb.
right click the file/personal gdb you just created, > new > Raster catalog.
right click the raster catalog you just created > load > Load raster datasets... load your datasets.
once completed in the toolbox, Data management tools > Raster > Raster dataset > raster catalog to Raster dataset

Hope this helps.

Re: Image problem when mosaic to raster

bernardfoong — Tue, 25 Sep 2012 03:06:08 GMT

Thanks for your reply.

I want to merge the DEMs that I downloaded and merge them into raseter in Arcgis. I intend to model the hydrological flow of the region.

I tried using the catelog and the gdb but i get the same result. Is there out solutions to smoothen the raster?

Thanks again.

Re: Image problem when mosaic to raster

ShaunLam — Tue, 25 Sep 2012 03:33:08 GMT

Are the DEMs overlapping ? is that what that black shade is ? the picture you uploaded is ridiculously small.

You can always try clipping the DEMs so that there is no overlap. Then running the process over.

If you have spatial analyst you can use the extract by mask tool to clip the DEM to a footprint.

Re: Image problem when mosaic to raster

bernardfoong — Tue, 25 Sep 2012 03:57:06 GMT

Hi shaun,

Many thanks for replying.

I am not sure why is there a dark shade. It happens when i mosaic all the DEMS together in Arcgis "moasic to to new raster" function.

How do I check if the DEMs overlap with it each other? I cant find the mask tool in spartial analyst. I am using version 9.3.

Can you give me a step-by-step approach as this is my first time using this software.

Thank you.

Re: Image problem when mosaic to raster

ShaunLam — Tue, 25 Sep 2012 04:03:14 GMT

It shouldn't matter if they are overlapping using mosaic to new raster or the raster catalog option.

What output extension are you trying to use ? Try use the default Esri Grid.

Re: Image problem when mosaic to raster

bernardfoong — Tue, 25 Sep 2012 04:12:08 GMT

The output I am trying to get a .aux file for the merged raster which i think is the default esri grid.

Re: Image problem when mosaic to raster

ShaunLam — Tue, 25 Sep 2012 04:16:16 GMT

So under raster dataset name with extension you aren't putting any extensions ?
How big are 2 of the tiles that show the black shade, are you able to send to me ?
shlam@golder.com.au

You can always try with smaller dataset to speed up the process.
Extract by mask is under spatial analyst tools, extraction, extract by mask, or just use the index or search tab under the toolbox and type it in.

Re: Image problem when mosaic to raster

ShaunLam — Tue, 25 Sep 2012 04:24:48 GMT

The output I am trying to get a .aux file for the merged raster which i think is the default esri grid.

I just re-read that, when you use the Mosaic to new Raster tool, Under "raster dataset name with Extension"
are you putting in there "filename.aux" ?

Re: Image problem when mosaic to raster

bernardfoong — Tue, 25 Sep 2012 04:32:57 GMT

under raster dataset name with extension I did not put any extensions. Is this the source of error?

Re: Image problem when mosaic to raster

EricRice — Tue, 25 Sep 2012 15:01:29 GMT

If the workspace is a folder, then you are writing an Esri GRID raster. If the workspace is a geodatabase, then you are writing a geodatabase raster. Neither would require you to specify an extension with the name.

Are you sure you aren't mixing different units of data when creating the mosaic? Often I see a line like yours when people mosaic data that is in feet with other data that is in meters. If you identify a pixel on either side of the black line is one ~3x bigger than the other?

Best,
Eric

Re: Image problem when mosaic to raster

bernardfoong — Tue, 25 Sep 2012 16:19:22 GMT

Hi Eric,

Thanks for your reply.

How do I check if the mosaic data is in feet or meter? Do I go to the Layer Properties of the raster and get the raster information under spatial reference? If so, the linear unit is given as meter (1.000000). If this is not the way to find it, how do i verify?

No, the black portion is 3 times smaller.

Thanks,
Bernard

Re: Image problem when mosaic to raster

EricRice — Tue, 25 Sep 2012 18:51:26 GMT

Bernard,

The linear unit has to do with coordinate system (X,Y) units, not the pixel (or Z) unit. You have to know this (local knowledge of terrain), if it is not in any metadata. There is nothing in the layer properties for the unit of the pixel - many rasters, like a landuse raster don't have a concept of units. Unless there is a giant cliff, two pixels on either side of the black seam shouldn't be 3x smaller/larger. It basically confirms that your are mosaicking data with different units. You have to pick a unit, convert any rasters that are not in that unit by multiplying by the correct unit conversion factor, then mosaic.

Best,
Eric

Re: Image problem when mosaic to raster

bernardfoong — Wed, 26 Sep 2012 03:43:11 GMT

Hi Eric,

Thank you for your explanation. It was clear and I understood it.

If that is the case, can you guide me on how to multiply the raster on a guide-by-step basis? I still new to the software.

Thanks,
Bernard

Re: Image problem when mosaic to raster

EricRice — Wed, 26 Sep 2012 14:43:50 GMT

You need 3d analyst or spatial analyst. There is a tool called Times in both of them. Input your raster and multiply it by the unit conversion factor. It's a really straight forward tool, but let me know if you have any questions.

Best,
Eric