Hi, As long as there is nobody on the list can make sense of the values for almost two weeks, I'd say it should be considered a bug and filed towards https://github.com/openstreetmap/openstreetmap-website/issues
чт, 11 янв. 2018 г. в 2:05, Bjoern Hassler <[email protected]>: > 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 <[email protected]> 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 <[email protected]> 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 <[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 >>> 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 < >>> [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]> 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 >>>>> >>>> >>> _______________________________________________ >>> dev mailing list >>> [email protected] >>> https://lists.openstreetmap.org/listinfo/dev >>> >>> >>> >> >
_______________________________________________ dev mailing list [email protected] https://lists.openstreetmap.org/listinfo/dev
