[OSM-dev] Scale of downloaded images seems to vary.

Bryan Housel bryan at 7thposition.com
Tue Jan 2 14:46:59 UTC 2018 

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 <bjohas+mw at 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 <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 <me at komzpa.net <mailto:me at 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+mw at gmail.com <mailto:bjohas%2Bmw at 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.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.
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