[OSM-dev] Scale of downloaded images seems to vary.
Tom Hughes
tom at compton.nu
Thu Jan 11 09:54:23 UTC 2018
Looks like we are using 90dpi actually. The key code is here:
https://github.com/openstreetmap/chef/blob/master/cookbooks/tile/templates/default/export.erb#L121
We project the bounding box from EPSG:4326 lat/lon to spherical mercator
which gives us coordinates on a projected sheet measuring roughly
40075016m on each edge (2 x PI x assumed earth radius).
We then divide by the scale factor and then convert metres to pixels by
dividing by 0.00028 which comes from:
1 / 39.701 / 90
Where we divide by 39.701 to convert from metres to inches and then by a
further 90 to convert to pixels at 90 dpi.
Tom
On 11/01/18 08:52, Tom Hughes wrote:
> Actually isn't the real problem here just needing to know what DPI is
> being being assumed by mapnik when rendering?
>
> IIRC it's 96dpi?
>
> Tom
>
> On 11/01/18 08:50, Tom Hughes wrote:
>> 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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>>>> dev at openstreetmap.org
>>>> <mailto:dev at openstreetmap.org>
>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>
>>>>
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>>>
>>>
>>>
>>>
>>>
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>>
>>
>
>
--
Tom Hughes (tom at compton.nu)
http://compton.nu/
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