[GIS] Extract Longitude and Latitude from Geotiff (ASTER 30m DEM)

geotiff-tifflatitude longitudematlab-mapping-toolboxqgis

my problem is apparently quite simple, but I cannot really find an easy answer: in QGIS I've created a raster layer by merging different GeoTiff images from the ASTER 30m DEM dataset.

The merging is ok: by moving the cursor I can see that the coordinates make sense and so the elevation.

What I'd like now is to extract the longitude and latitude informations from each pixel (generating a matrix the size of the layer with Lat values and one with Lon values), but I really don't know how to do it.

I also tried to import the image in MATLAB using the geotiffread function, but I cannot extract such information…

Thanks to all!

Best Answer

I suggest not to create such a matrix, which could be quite inefficient, but if you really need this, you create an ascii file with Long, lat, and value as columns, (which you can import in Matlab.)

gdal_translate -of XYZ input.tif output.asc

gdal is also available from qgis as a plugin (with a GUI), but you can use it directly as a command line.

Note that you can find your coordinates directly based on the row and column number. The easiest case is a starting from a matrix that is already in long/lat, in other words a regular grid in Lat/long coordinate system.

so you need to extract the upper left corner coordiantes, and the lat/long can be computed based on the index of you element

lat = upperLeftLat - Ysize*row

long = upperLeftLong + XSize*column

If you are not in this ideal case, you need to account for the rotation of the matrix and, potentially, the projection. But this can also be done "on the fly" without creating a new matrix (although if you need to reproject your data, it is better to do it first).

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[GIS] locating latitude-longitude point in a piste-map with matrix transforms

If the image is in the least realistic--and it appears to be--then it represents a projection from a portion of Euclidean three-space onto a portion of two-space. We cannot expect this projection to have a nice mathematical formula, because it is likely the artist has introduced irrregular small distortions, but we can expect its formula to somewhat approximate formulas for such projections. These formulas tend to be linear or projective and they also tend to be simpler when Cartesian coordinates are used.

Therefore, what you should consider doing at the outset is this:

Convert all (lat, lon, elevation) values to geocentric (x, y, z) values.
Associated with each (x, y, z) value is a pair of image coordinates (u, v). We probably lose little by estimating u and v independently of each other, which reduces the problem to one of interpolating samples of a scalar function. The previous considerations suggest that you seek interpolating functions that locally are of the form
```
u = a x + b y + c z + d
```
(linear) or even
```
u = (a x + b y + c z + d) / (a' x + b' y + c' z + 1)
```
(projective) and similarly for v. This reduces the problem to one of performing an interpolation in three dimensions, which is sufficiently routine to give you access to a large body of methods and software.

Alternatively, you might consider triangulating the (x,y,z) points and interpolating their associated (u,v) values separately across the triangles; even a linear interpolation across each triangular face could do a pretty good job. This is a 3D analog of what ESRI's 3D Analyst does in two dimensions with TINs.

Please note, though, that unless you have the elevation of a given location, you cannot hope to plot it accurately on this map: changing the elevation will create a nearly vertical line on the map, whence your location could be plotted practically anywhere along this line. With considerable work you could solve this problem for points on the ground by first digitizing the evident discontinuities in the inverse projection--that is, representing the outlines of mountain ridges as polylines--and constructing a local inverse interpolator (respecting these breaklines) that associates to each (u,v) on the image a location (x,y,z) in the world. You could exploit this to identify which point along any vertical line has a (lat,lon) close to the one you started with.

[GIS] Latitude – Longitude bounds from GeoTIFF info

Sure,

Firstly, open it in QGIS, and then in the menu at the top go to Raster->Miscellaneous->Information.

Select the layer in the "Input-file" drop-down box (if it's not already there) then press OK, and it will create a load of text relating to information about the layer in a box called "Raster Info".

If you scroll down this text, you will get to a line that says "Corner Coordinates", the following 4 lines look like this:

Upper Right ( 5486108.843, 5015161.555) ( 14d49'22.49"E, 45d16'51.06"N)

and describe the coordinates of each corner (the geographic bounds) (in my example, given as both projected and geographic coordinates).

Is that what you're after?

Note: if you can't see any part of Raster->Miscellaneous->Information in the menu, go to Plugins->Manage and Install Plugins, and make sure that the box next to "GDAL Tools" is ticked, this will make it appear

Best Answer

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[GIS] Latitude – Longitude bounds from GeoTIFF info

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