Bjoern, maybe the geo functions used in iD might be a helpful reference: https://github.com/openstreetmap/iD/blob/master/modules/geo/geo.js <https://github.com/openstreetmap/iD/blob/master/modules/geo/geo.js>
The numbers I got from comparing the bbox sizes are pretty close to your numbers. bbox1 = [[24.123255,49.250507], [24.234286,49.367924]] dLat1 = bbox1[1][0] - bbox1[0][0] > 0.11103100000000055 iD.geoLatToMeters(dLat1) > 12359.91438226802 dLon1 = bbox1[1][1] - bbox1[0][1] > 0.11741700000000321 iD.geoLonToMeters(dLon1, (bbox1[1][0] + bbox1[0][0])/2) > 11884.145336433623 (image1 is 11.884 km x 12.359 km) bbox2 = [[48.632228,-101.369133], [48.691074,-101.251717]] dLat2 = bbox2[1][0] - bbox2[0][0] > 0.05884600000000262 iD.geoLatToMeters(dLat2) > 6550.706755221268 dLon2 = bbox2[1][1] - bbox2[0][1] > 0.11741600000000574 iD.geoLonToMeters(dLon2, (bbox2[1][0] + bbox2[0][0])/2) > 8604.30156213755 (image2 is 8.604 km x 6.550 km) Bryan > On Jan 1, 2018, at 6:56 AM, Bjoern Hassler <[email protected]> wrote: > > Hi Darafei, dear all, > > Thanks, but I still cannot get this to work. > > I've now calculated real_scale = mapnik_scale / cos(lat), and used the > real_scale, to calculate: > > pixels * (72/2.54 pixels/cm) * real_scale = real_world_dim > > However, there's still a latitude-dependent discrepancy (see below). I could > try to fit that to latitude, to see what the formula is, but I'm hoping > somebody has the answer (or can let me know what I got wrong!) > > Happy new year! > Bjoern > > Example 1: > http://www.openstreetmap.org/#map=13/24.1788/49.3092 > <http://www.openstreetmap.org/#map=13/24.1788/49.3092> > bbox = [24.123255,49.250507; 24.234286,49.367924] > bbox size in degrees (lon, lat) = 0.117416, 0.111031 > Pixels ('Image ... at'): 1945 x 2016; > mapnik_scale 1 : 24000; real scale 1 : 26308 > Image dim (1 : 26308, 72dpi): 686 mm x 711 mm > Real world dim (1:1, from pixels): 18.051 km x 18.71 km > Real world dim (1:1, latlon): 11.911 km x 12.346 km > Ratio (dim pixels/ dim latlon): 1.516 ; 1.515 > > Example 2: > http://www.openstreetmap.org/#map=13/48.6617/-101.3104 > <http://www.openstreetmap.org/#map=13/48.6617/-101.3104> > bbox = [48.632228,-101.369133; 48.691074,-101.251717] > bbox size in degrees (lon, lat) = 0.117416, 0.058846 > Pixels ('Image ... at'): 1945 x 1476; > mapnik_scale 1 : 24000; real scale 1 : 36336 > Image dim (1 : 36336, 72dpi): 686 mm x 521 mm > Real world dim (1:1, from pixels): 24.932 km x 18.92 km > Real world dim (1:1, latlon): 8.624 km x 6.543 km > Ratio (dim pixels/ dim latlon): 2.891 ; 2.891 > > > > On 31 December 2017 at 18:59, Darafei "Komяpa" Praliaskouski <[email protected] > <mailto:[email protected]>> wrote: > 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] > <mailto:bjohas%[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 > > <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 > <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] <mailto:[email protected]> > https://lists.openstreetmap.org/listinfo/dev > <https://lists.openstreetmap.org/listinfo/dev> > > _______________________________________________ > dev mailing list > [email protected] > https://lists.openstreetmap.org/listinfo/dev
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