[OSM-dev] Scale of downloaded images seems to vary.
Tom Hughes
tom at compton.nu
Thu Jan 11 08:50:16 UTC 2018
Please don't - it has nothing to do with the web site code.
Please go and read about projections instead.
Tom
On 11/01/18 08:21, Darafei "Komяpa" Praliaskouski wrote:
> 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 <bjohas+mw at gmail.com
> <mailto:bjohas%2Bmw at gmail.com>>:
>
> 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+mw at gmail.com
> <mailto:bjohas+mw at 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 <bryan at 7thposition.com
> <mailto:bryan at 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+mw at gmail.com <mailto: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
>> 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 <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
>> - "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.
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>>
>>
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--
Tom Hughes (tom at compton.nu)
http://compton.nu/
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