Elevation grid export problem

Michael_GRMichael_GR Global Mapper UserPosts: 10
edited September 2012 in Elevation Data
I'm trying to export a large area as a grid of GeoTiff files.
The area in question is TExas and some surrounding regions and my input is downloaded from the net - USA topo maps and in some areas ASTER GDM data. I'm exporting a grid of 1-degree tiles, about a hundred in all.
Anyway, what I wanted to know was what what determines the time it takes to export a file. The first few tiles took about a minute to export. The second row took 2-3 minutes per tile. The row after that took more than 10 minutes.
Is there a way to find out why is there such a difference in rendering time? is it some memory problem?

another question - one of the tiles, when viewed in an image viewer, looks like it's completely black, very different from its neighbors. Is this natural or a problem? should I try to export it again?

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2012
    If you have a large number of GeoTIFF files loaded try instead creating a map catalog (File->Create New Map Catalog) from them and then exporting from that. The map catalog will allow the most efficient access method to be used for all of the GeoTIFF files, resulting in more consistent export times, assuming those are the only source. Of course any part of your export that has to pull data from online sources may also be variably slow depending on the network speed.

    Are you exporting to a terrain GeoTIFF? If so this likely won't show up in most image viewers as it will be a 16-bit or 32-bit file with actual elevation values. Does the file load and display back in Global Mapper?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • Michael_GRMichael_GR Global Mapper User Posts: 10
    edited September 2012
    Thanks for the quick reply! but I don't have any GEOTIFFs loaded - all I have is downloaded online data sources - united states elevation data and ASTER GDEM worldwide elevation data. Yes, I'm exporting to GEOTIFF, and looking at the Windows Task Manager I can see the CPU isn't working too hard. So is it just a matter of download speeds from the web? Is there some way of pre-loading all of the data before export?

    As for that black file, all of the files save for that one are perfectly visible in my viewer (IrfanView). but when I load it and the surrounding tiles back in GlobalMapper it all looks fine so it's probably not a problem.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2012
    Oh yeah it is probably just the download times them. Often servers will throttle clients that are downloading too much, so it will go fast at first, then slow down. Global Mapper does cache the data so future access to the same area at the same resolution will be much faster. The initial download is slow though.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
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