[GraphHopper] finding correct offline tile folder to use

[Paulo Tubbert-Semiao]

Hi Jamie,

I've never used this personally but I previously came across this on the
OpenStreetMap Wiki:

n = 2 ^ zoom
lon_deg = xtile / n * 360.0 - 180.0
lat_rad = arctan(sinh(π * (1 - 2 * ytile / n)))
lat_deg = lat_rad * 180.0 / π


The full page is here:

http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames

I hope this helps!



On Tue, Feb 11, 2014 at 2:32 PM, Jamie Morken <jmorken at shaw.ca> wrote:

> Hi,
>
> I am making some code to view offline tile folders, and need to find the
> correct
> folder and file using the GPS coordinates.  I "reverse engineered" an
> offline tile
> format, and noticed that the folders are determined by longitude and the
> files in each
> folder are determined by latitude.  I came up with this formula for
> longitude to find
> which folder to access given a longitude from GPS:
>
> correctLongitudeFolderToUse = (inputLongitude+180)*((2^currentScale)/360)
>
> so if the input longitude is say -120 degrees (west coast) then at a scale
> of 16 the formula works out to:
>
> correctLongitudeFolderToUse =(-120+180)*((2^16)/360)
>
> correctLongitudeFolderToUse = 10922.66
>
> This works properly, if I go to the folder with name 10922 the longitude
> is correct.
>
> I have an error for finding which file to access for latitude though.  I
> use this
> formula for latitude:
>
> correctLatitudeFileToUse = (-inputLatitude+90)*((2^currentScale)/180)
>
> so if the input latitude is 40 and the scale is again 16 this is the
> result:
>
> correctLatitudeFileToUse = (-40+90)*((65536)/180)
>
> correctLongitudeFolderToUse = 18204.44
>
> so in this case it would say the file name should be 18204.jpg in the
> folder 10922,
> however the latitude is incorrect.  I think it is a scaling error since
> for the two poles
> and equator the formula works correctly, and also for 30degrees latitude I
> think it may
> work correctly.  I think I need a sin or cos term in the formula but not
> sure.
>
> Any ideas on how to fix the latitude formula?
>
> cheers,
> Jamie
>
>
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