I am using the freely available dtm data from the OS UK website (Land-form panorama and its in the ASCII format). when you download the data from the website, the dtm is found in different subfolders (labelled according to the national grid squares, e.g. NN, NO, HY etc.). within each folder there are number of ASC files (which looks like tiles when you view on ArcGIS - I am not sure what the correct technical term is). My sites are spread out across an area of about 23 national grid squares - so there is a large number of ASC files that needs to be added. When I try to open the dtm data on ArcGIS 10.1 I encountered a problem, I can only open the ASC files from about 4 national grid squares before the program freezes and shuts down. The primary focus of my study is to conduct viewshed analysis for my sites. when I add the various ASC files and try to conduct a viewshed analysis it looks like this:

enter image description here

The problem is my sites are in clusters and some sites are found spread out into the neighbouring tiles. it looks like when I try to do the viewshed analysis it doesn't take into account the sites that are found in the nearby tiles - which is a huge problem for me.

I have been told to create a raster mosaic but when I tried doing that it still comes up looking like separate tiles:

enter image description here

Am I doing something wrong? I am also having trouble performing the viewshed analysis here (although I can perform a hillshade), I was able to do it before I created the mosaic dataset. The other problem is I still have to do the hillshade and viewshed analysis one asc file at a time. is there a way around this?

one other thing - when I added the raster data to create a mosaic dataset - it comes up as empty green squares - the terrain only becomes visible if I calculate the statistics for each individual asc file - it remains blank when I try to do a batch statistics. I am not sure what I am doing wrong, if you could help me it will be greatly appreciated!

  • When you say "before the program freezes and shuts down" it makes me think that you should contact your local Esri Technical Support and/or see if you observe the same symptoms when you repeat the same steps on another machine. To encourage people here to repeat your steps I recommend that you edit your Question to list each one precisely.
    – PolyGeo
    Commented Jun 3, 2014 at 23:55
  • Related/duplicate: gis.stackexchange.com/questions/66846/…
    – Chris W
    Commented Jul 1, 2014 at 20:49

3 Answers 3


It looks like you're creating raster mosaic datasets. This will be ok for hill shades but I think you will have to creat a raster mosaic to create a viewshed over your entire project. A raster mosaic datasets is like an image catalog that references original files. A raster mosaic takes all source images and merges them into one complete image. The latter will certainly work for your viewshed.

  • I didn't know that there was a difference. can you tell me how to create a raster mosaic rather than a raster mosaic dataset? I would really appreciate any assistance!
    – user19622
    Commented Jul 27, 2013 at 4:18

In arc catalog you right click in the location you want to build the mosaic, then choose new, then pick raster mosaic. Fill in the necessary details and hit ok. Then load your images. Heres's a tip: do not create pyramids or calculate statistics while loading (uncheck those boxes). After the loading is complete then you calculate stats and build pyramids his will make both processes faster to complete.


  • Thanks for that. I have done what you suggested but it still comes up as empty green squares, I can't see the terrain. is there any way to fix this?
    – user19622
    Commented Jul 31, 2013 at 22:15

ASCII is an inter-change text file format. ArcMap will behave better if you convert these inter-change files into ESRI grids then mosaic those as described above. You can make a simple model to batch convert all these ASCII files, this thread shows an example.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.