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-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 >>> 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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