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How to find datum / projection info of a raster map?

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How to find datum / projection info of a raster map? Empty How to find datum / projection info of a raster map?

Post  Breto on Thu Jan 17, 2013 12:20 pm

I was wondering if it is possible to deduce datum and projection type of a map have meridian and parallels drawn on it.

For instance this one: http://carte.f-aero.fr/

The datum is most likely WGS84.
The projection is unknown, but meridians are all exactly vertical and parallels are all exactly horizontal, suggesting a cylindrical projection. In addition, expressed in pixels, meridians have a constant spacing while parallels have regularly increasing spacing.

Is it possible to find what projection was used with these observations? Can the spacing law between parallels yield useful information?

Thanks,
Breto



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Post  orux on Thu Jan 17, 2013 9:27 pm

Breto wrote:I was wondering if it is possible to deduce datum and projection type of a map have meridian and parallels drawn on it.

For instance this one: http://carte.f-aero.fr/

The datum is most likely WGS84.
The projection is unknown, but meridians are all exactly vertical and parallels are all exactly horizontal, suggesting a cylindrical projection. In addition, expressed in pixels, meridians have a constant spacing while parallels have regularly increasing spacing.

Is it possible to find what projection was used with these observations? Can the spacing law between parallels yield useful information?

Thanks,
Breto



Try with the Mercator projection.
It has the features you mention. And is that used by Google, Microsoft, ... maps.

orux


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Post  steed on Fri Jan 18, 2013 11:30 am

Hi,

I would recommend looking into the URL of the tiles in such pages. In Firefox, it can be done using right-click -> View page info -> Media

There you can find that a typical URL of a tile would be something like:

http://carte.f-aero.fr/tilecache-2.11/tilecache.cgi?LAYERS=oms2012&FORMAT=image%2Fpng&TRANSPARENT=TRUE&REPROJECT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG%3A900913&BBOX=126973.79416841,5739201.2923134,166109.55264497,5778337.05079&WIDTH=256&HEIGHT=256

A Google search for "EPSG%3A900913" would tell you that EPSG:9000913 is a Spherical Mercator projection.

Good Luck,
Steed

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Post  orux on Fri Jan 18, 2013 7:04 pm

steed wrote:Hi,

I would recommend looking into the URL of the tiles in such pages. In Firefox, it can be done using right-click -> View page info -> Media

There you can find that a typical URL of a tile would be something like:

http://carte.f-aero.fr/tilecache-2.11/tilecache.cgi?LAYERS=oms2012&FORMAT=image%2Fpng&TRANSPARENT=TRUE&REPROJECT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG%3A900913&BBOX=126973.79416841,5739201.2923134,166109.55264497,5778337.05079&WIDTH=256&HEIGHT=256

A Google search for "EPSG%3A900913" would tell you that EPSG:9000913 is a Spherical Mercator projection.

Good Luck,
Steed

Hmmm,

it is a WMS...

http://carte.f-aero.fr/tilecache-2.11/tilecache.cgi?SERVICE=WMS&VERSION=1.1.1&REQUEST=GetCapabilities&STYLES=&

But only supports <SRS>EPSG:900913</SRS> not supported by OruxMaps WMS tool.

orux

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Post  Breto on Sun Jan 20, 2013 7:50 pm

With the projection info you gave me I have been able to make a correct offline map out of it.

Thanks for your help.

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Post  steed on Mon Jan 21, 2013 12:00 pm

Great!

Can you describe the flow and tools you've used?
I have a similar issue I was unable to solve.

Thanks,
Steed

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