Images are in Spherical Mercator EPSG:3857 projection, so linear scale is off by cos(lat).
On Sun, Dec 31, 2017, 20:07 Bjoern Hassler <[email protected]> wrote: > Dear friends, > > I'm trying to make sense of the scales for map images downloaded from OSM. > For the download, you can choose the scale, and I had assumed that I could > use this to convert to an actual map scale. > > The downloaded png/jpg etc seem to be at 72dpi. I had assumed I could just > convert pixels at 72dpi to actual dimensions (using the scale). > > However - as far as I can tell - this doesn't work. Maybe I've made a > mistake somewhere, but the dimensions calculated from > > - "feature in pixels" / (72/2.54 pixels/cm) * scale = "feature size" > in cm > - lat-lon (e.g. bounding box provided) > > Doesn't match. Moreover, the difference doesn't seem to be a constant > offset or ratio, but possibly latitude dependent. > > Maybe the scale offered during download is not meant to be a geographic > scale? Maybe I've misunderstood something? > > There are two worked examples below, that show the issue. > > Any thoughts? > Bjoern > > (and a Happy New Year!!) > > > *Example 1:* > > I had a look for long straight roads ... (Trivia: > http://www.dangerousroads.org/rankings23/3759-the-10-longest-straight-roads-in-the-world.html > - "Located in the heart of Saudi Arabia, the Highway 10 is 120 miles > (193km) stretch of straightness. This asphalted road links Haradh and Al > Batha. It’s a straight road running right through the desert for 2 h 1 > min.") > > - Open 'share', > - set scale to 1:50000, > - adjust view port so that "Image will show standard layer at 932x..." > - Go here: http://www.openstreetmap.org/#map=13/24.1349/49.3083 > > On the map, there's a road (East/West), with two turn-off: First, a power > line at the Eastern edge (running North/South). In the west, there are two > turn-off, the second (straight one) being 11.9 km from the power line > (according to JOSM). In the image, you've got those right at the edges. > From the bounding box (hidden fields), I calculate 11.62km. Given that the > roads are just showing either side of the image, that's bang on. > > Now download PNG, which will have with 932. I am assuming I have a PNG > (72dpi = 28.35 dots per cm), at scale 1:50,000. I calculate: > > 932 pixels / (72/2.54 pixels/cm) * 50000 = 16.4 km. > > So there's a difference between the dimensions calculated from the pixels > and the distance calculated from lat/lon. > > *Full details for Example 1:* > > Z/L/L #13/24.1727/49.3090 > bbox = [24.119651808471247,49.249992370605476 -> > 24.22567631717543,49.368095397949226] > Pixel dim: 939 x 924; > Natural image dim (72dpi): 331 mm x 326 mm, 1 : 50000 > Real world dim (from pixels): 16.563 km x 16.298 km, 1 : 1 > Real world dim (latlon): 11.981 km x 11.789 km, 1 : 1 > Ratio: 1.382438861530757 ; 1.3824751887352615 > > *Example 2:* > > Another example from the above list: > > Z/L/L #13/48.6536/-101.3485 > bbox = [48.615207636211146,-101.44741058349611 -> > 48.69198023486001,-101.24965667724611] > Pixel dim: 1572 x 924; > Natural image dim (72dpi): 555 mm x 326 mm, 1 : 50000 > Real world dim (from pixels): 27.728 km x 16.298 km, 1 : 1 > Real world dim (latlon): 14.526 km x 8.537 km, 1 : 1 > Ratio: 1.908853091009225 ; 1.909101557924329 > > The distance (along the highway) from the turnoffs to Undip / Lansford > airstrips is 8.1km in JOSM. So the latlon calculation is correct. However, > the dimension calculated from the pixels isn't. > _______________________________________________ > dev mailing list > [email protected] > https://lists.openstreetmap.org/listinfo/dev >
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