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

Bjoern Hassler bjohas+mw at gmail.com
Mon Jan 1 11:56:17 UTC 2018


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
>>
>
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