Global Mapper v25.0

converting DTM's to asc files

katewalker
katewalker Global Mapper User
edited February 2011 in Raster Data
Hi,
Is there a limit to the size of .asc files that GM can open? I have DTM's of catchments that I need to convert to .asc files to model with them. I opened the orginal DTM tiles in ArcMap and cut out the shape of my catchment to produce a new DTM. This I then opened in GM (no probs) and exported it to an .asc file type, again no probs. I then wanted to make sure the conversion had worked so I asked GM to open the newly created .asc file and up popped an error message 'unknown error loading data file'. I've looked at the .asc in Surfer and it looks fine!! Any help or suggestions gratefully received!

Comments

  • global_mapper
    global_mapper Administrator
    edited February 2011
    What version of Global Mapper are you using and how large was the generated .asc file?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • katewalker
    katewalker Global Mapper User
    edited February 2011
    Hi Mike,
    One of the catchment DTM's was a total of 843,923KB as a raster which made an .asc of 1.57GB - they are large becasue they cover large areas (river catchments). The catchment fell across more than one DTM tile so in ArcMap I mosaiced the two catchment halves together. The reason I thought it might have been the size was that when I converted each half separately to .asc and back, the smaller of the two (237,356KB) was fine whereas the larger (606,577KB) came up with the error message when I tried to convert from an .asc in GM. I am using GM 9.

    Thank you
    Kate
  • global_mapper
    global_mapper Administrator
    edited February 2011
    Kate,

    In GM9 .asc format files are loaded entirely into memory, so you are likely unable to load it due to size. In GM12 (the latest version) large .asc files are extracted to an on-disk binary file, then read on the fly from there to minimize memory usage. In practice the .asc format isn't ideal for very large data sets since you can't directly access the data like from a binary format.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com