[OSM-dev] Scale of downloaded images seems to vary.
Tom Hughes
tom at compton.nu
Thu Jan 11 08:52:06 UTC 2018
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.
>>> _______________________________________________
>>> dev mailing list
>>> dev at openstreetmap.org <mailto:dev at openstreetmap.org>
>>> https://lists.openstreetmap.org/listinfo/dev
>>>
>>>
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>>> dev at openstreetmap.org <mailto:dev at openstreetmap.org>
>>> https://lists.openstreetmap.org/listinfo/dev
>>
>>
>>
>>
>>
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>
--
Tom Hughes (tom at compton.nu)
http://compton.nu/
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