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

Thu Jan 11 08:21:08 UTC 2018

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>:

> 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> 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> 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> 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> 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> 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
>>>>> dev at openstreetmap.org
>>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>>
>>>>
>>> _______________________________________________
>>> dev mailing list
>>> dev at openstreetmap.org
>>> https://lists.openstreetmap.org/listinfo/dev
>>>
>>>
>>>
>>
>
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