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

Sun Dec 31 18:59:22 UTC 2017

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.
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>
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