Sunday, November 13, 2011

Printing customised top maps from LINZ

Land Information New Zealand (LINZ) has topographical maps that cover New Zealand available for free download.

I am planning a trip over the Tongariro Crossing, and wanted to print a topo map that covers the walk. Unfortunately, as so often happens, the walk traverses two maps. To solve this problem, I decided to merge two maps and then print only the appropriate part of the map.

To merge the maps, the simplest method was to download the two maps in GeoTiff format and merge them using the GDAL tools, which are compiled by Frank Warmerdam into a handy Windows package:

gdal_merge -v -q -o merged.tif -of GTiff BH34_GeoTifv1-01.tif BH35_GeoTif_1-00.tif

The next step was to crop the appropriate part of the map so that the scale was preserved when I printed it on A3 paper. A key point to note is that the original LINZ GeoTiff is set to 300 DPI or 118.11 pixels/cm. In the drawing tool I used (Paint.NET), I was able to set the DPI to 300 and then set a crop area of 42 x 29.7 cm to select the area I wanted. Bear in mind that many printers are not able to print onto the full paper size, so a smaller area may be required to preserve the scale.

Note that the -v -q arguments were required for the version of FWTools that I downloaded due to a known issue in GDAL.


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