GLIMS shapefile flattening tool


This is an on-line tool to "flatten" shapefiles. Some people have digitized glaciers in such a way that internal rock outcrops (nunataks) have been represented by "holes" in the glacier polygon. In the shapefile itself, these "holes" are sub-parts to the main glacier polygon, and share attributes with it. That is, the glacier perimeter and the holes (rock outcrops) have only one entry in the attribute table.

The GLIMS Data Transfer Specification, however, requires that internal rock outcrops be represented by their own top-level polygons, so that they can be individually attributed. This tool converts shapefiles with glacier polygons containing holes into shapefiles where the holes have been made into top-level polygons. These new polygons inherit all the attributes of their parents, except that the "category" attribute is set to "intrnl_rock". For best results, the input shapefile(s) should already have the attribute "ID" populated with GLIMS glacier IDs, and the attribute "category" populated with "glac_bound". (As of 2008-06-25, the "category" attribute will be added to the file automatically if it is not present.)

In some cases, software produces shapefiles where the top-level polygons and "holes" are mixed up, and the outer-most ring (polygon) is actually a sub-part in the shapefile. If this is true for your shapefile, the flattened result will identify the the glacier boundary as "internal rock", and one of the internal rock boundaries as "glacier boundary". If this happens to you, try clicking on the checkbox below. This will force the top-level polygon to be identified by vertex count, rather than the structure of the shapefile. The ring with the most vertices is assumed to be the glacier boundary polygon. (In the future, this code may be enhance to check for containment instead.)

The uploaded file should be a zip, tar, or gzipped tar archive containing one or more 'segments' shapefiles (.shp, .shx, and .dbf files). The archive file may contain subdirectories. If all goes well, after you press "Submit", you will have a link for downloading a zip archive of the flattened version(s) of the shapefile(s).

This tool is no longer necessary for GLIMS data contributors, as we perform this operation as necessary here at NSIDC. If you are interested in using this tool, please write us for the code.