Generating Scale Bar for Raster Image Data

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My goal is to create a scale bar for a printed map at 1:1,000,000 scale.

The input dataset is a Mercator projected geotiff. Using GDAL it is possible to get the spatial reference information and dataset metadata. One component of the metadata is pixel resolution (at the equator). Assume each pixel is ~4km square.

The scale bar will be simplistic, and the goal is determine the printed length of the scale bar at 100km.

I am struggling to define the ratio between printed size and visualized size. For example, at 1:1000000, I can define the unit as cm and know that a 10cm scale bar is required. But this needs to be related back to the map and I am completely missing that (basic) linkage.

  • I am simplifying and assuming the scale bar only needs to be accurate at the equator.

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