Deviants
Posted: Wed 19 Jun, 2024 5:53 pm
Playing with UTM grid overlays for tiled mosaic maps (like Basecamp Topo) got me thinking about these maps. The spatial.nsw Topo mosaic is a good test case, because it comes with a UTM grid. The mosaic seems to be built from mapsheets in a UTM projection, but the mosaic is not in a UTM projection. A (1 km) UTM grid is about 10% larger on screen in the South of the state than in the North. And some grid ‘squares’ in the middle are noticeably rectangular. I am guessing that like other mosaic maps (Google Earth, Bing, Garmin?) it uses a variant on spherical Mercator rather than UTM. It is probably inevitable that there are more distortions in a state-wide mosaic than in a 1:25000 map sheet. Maybe we hikers should be less lazy and build a custom map from a select few sheets (pdf or tif) rather than using a small chunk from a big mosaic when we hike near map sheet boundaries? (Light-weight hikers only want to carry one map).
Tom probably knows more about this than I do. His program https://maps.ozultimate.com/archive/08_print/# for NSW has a scale bar that changes size with the grid spacing; but the printed area does not change, so users will get a slightly different map scale depending on latitude.
I don’t care much about scale when there is a UTM grid: as it gives me a 1 km spacing. But (for a backup map) I do care about magnetic declination. This varies across Australia, but those who assume 10º E in the Eastern stares will probably be OK. Those who ignore magnetic declination (assume 0º everywhere) may be in strife. Grid-magnetic variation is more ... variable. Basecamp shows magnetic declination if you choose File>Print, but if you clip and paste (from Basecamp or another mosaic) you need to take care of it yourself; eg write it on your map. Maybe it is possible to code in an arrow or statement of magnetic declination in each map printed from programs like Tom’s?
Tom probably knows more about this than I do. His program https://maps.ozultimate.com/archive/08_print/# for NSW has a scale bar that changes size with the grid spacing; but the printed area does not change, so users will get a slightly different map scale depending on latitude.
I don’t care much about scale when there is a UTM grid: as it gives me a 1 km spacing. But (for a backup map) I do care about magnetic declination. This varies across Australia, but those who assume 10º E in the Eastern stares will probably be OK. Those who ignore magnetic declination (assume 0º everywhere) may be in strife. Grid-magnetic variation is more ... variable. Basecamp shows magnetic declination if you choose File>Print, but if you clip and paste (from Basecamp or another mosaic) you need to take care of it yourself; eg write it on your map. Maybe it is possible to code in an arrow or statement of magnetic declination in each map printed from programs like Tom’s?