Huge File Workarounds

If you have a very large file, it takes a long time for operations, and you only need part of the data.

File Type MICRODEM native solutions QGIS solutions If you installed GDAL, you should have QGIS as well
Vector shapefiles

Open a map, Subset & zoom to the region you want. You have three choices:

  1. Subset DB before opening it in MICRODEM
    • File, Import, Shapefile after subset to match map extent.  You can adjust the map extent first to cover your area of interest.
    • This uses GDAL/OGR, with possible problems.
    • The file will be loaded.   It will be in a subdirectory of the directory with the initial shapefile.
  2. Filter the file ASAP
    • Open the shapefile.
    • Abort drawing if it is taking too long
    • Filtering options
    • Report, Shapefile export for line/areas, or Export DBF Table for points.
      • current map extent option
      • current filter option
    • Open the new file and close the original file.
  3.  Open in QGIS, extract just the portion you need, and open that.  If the file is too big for MICRODEM, it is likely to be very slow in QGIS as well.

QGIS will open the files rapidly. 

Use QGIS to export/subset, noting possible problems.

  • Option 1, when you need only part of the map extent
  • Option 2, when you can afford a lower grid resolution
  • Option 3, when the input file is too big to open (say a compressed Geotiff; probably has to be geographic)
    • Open a map, Subset & zoom to the region you want.
    • File, Import, Raster after subset to match map extent
  • Option 4: open in QGIS, and subset there.
QGIS will open the files rapidly. 
Use QGIS to export/subset.
The export option produces a smaller file that works easily.

QGIS file subset geographically

GDAL/OGR shapfile problems when subsetting geographically:  The export option has issues we have not gotten around for some shapefiles (e.g. NASA MODIS fires).  These appear to be in GDAL/OGR, because the same problems are in MICRODEM and QGIS which both use GDAL.  Other files, like TIGER shapfiles, work fine.

Last revision 2/3/2021