Dear friends, I was just wondering whether anybody else had any thoughts on this? Any tips on making sense of the mapnik_scale would be greatly appreciated!
Bjoern On 3 January 2018 at 17:21, Bjoern Hassler <bjohas...@gmail.com> wrote: > Hi Bryan, hi Darafei, > > That's helpful, thanks. So we know that the calculation from the bbox is > correct. > > However, I guess we don't know about how pixels translate to real-word > dims? (Or, equicvalenly, how pixels relate to the lat-lon extent.) > > Thanks! > Bjoern > > On 2 January 2018 at 14:46, Bryan Housel <br...@7thposition.com> wrote: > >> Bjoern, maybe the geo functions used in iD might be a helpful reference: >> 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 <bjohas...@gmail.com> 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 >> 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 >> 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 < >> m...@komzpa.net> 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 <bjohas...@gmail.com> 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.dangerousr >>>> oads.org/rankings23/3759-the-10-longest-straight-roads-in-th >>>> e-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 >>>> dev@openstreetmap.org >>>> https://lists.openstreetmap.org/listinfo/dev >>>> >>> >> _______________________________________________ >> dev mailing list >> dev@openstreetmap.org >> https://lists.openstreetmap.org/listinfo/dev >> >> >> >
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