[GIS] Extract row & column numbers of a raster at points in QGIS

qgisrastersampling

Is there a tool in QGIS that will extract the row and column numbers of a raster at a set of points? All I have been able to find is the following:

Raster Analysis > Sample raster values
(Plugin) Point Sampling Tool
GRASS > Vector > v.sample

None of these return the row & column number of the point, as far as I have been able to tell.

I also found this question, but it is rather in-depth and I was hoping to find a tool which could do it automatically if possible.

Best Answer

One way to do this is with the gdallocationinfo utility:

micha@TP480:GTOPO30$ gdallocationinfo -geoloc gt30e020n40.tif 31 29
Report:
  Location: (1320P,1320L)
  Band 1:
    Value: 15

You enter the X-Y the -geoloc parameter, and X-Y coords at the end, and it returns the Pixel and Line numbers (together with the value at that pixel)

Related Solutions

[GIS] How to extract all distinct values from a PostGIS Raster

There is an easier and more efficient way of doing this. Works for both PostGIS 2.1 and 2.0. Just use the ST_ValueCount function.

http://postgis.net/docs/manual-2.1/RT_ST_ValueCount.html

that will give you both the pixel value and number of pixels that have that value.

So would be for your case

SELECT DISTINCT (pvc).value
 FROM (SELECT ST_ValueCount(rast) As pvc
   FROM myrastertable) As f;

If you also want count of pixels that have each value, do

SELECT (pvc).value, SUM((pvc).count) As total
 FROM (SELECT ST_ValueCount(rast) As pvc
   FROM myrastertable) As f
    GROUP BY (pvc).value;

You might also want to throw in an order by (pvc).value to sort by the pixel value.

[GIS] Receiving NULL values from Point Sampling Tool in QGIS

In your (excellent) description of the problem, one thing worries me: your use of the "Set Layer CRS". This should almost never be used, and often causes trouble. THis option has only one (rarely needed) purpose: If you load a layer which:

Is lacking a CRS definition AND
You know in advance what the correct CRS is

then you can attach the correct CRS definition to the layer.

But, if both data layers were from the start in the same CRS you should NOT need to "Set Layer CRS".

Can you verify if "On the Fly" reprojection is active? If not, activate it, then check if the layers still overlap.

=== Continuation ===

The next thing I would try: do the same procedure using GRASS. Here are the steps:

Start GRASS and create a new LOCATION based on the EPSG:2452 CRS
Import both your flood depth raster and the building centroids into this LOCATION using the modules r.in.gdal and v.in.ogr
Get the raster values at each point location. First add a column to hold the raster values: v.db.addcol map=<centroids> column="flood_depth DOUBLE PRECISION" or alternatively, if there is no database yet for the centoids v.db.addtable map=<centroids> column="flood_depth DOUBLE PRECISION"
Now run v.what.rast vector=<centroids> raster=<flood_depth_raster> column=flood_depth

You should now have an attribute column in the centoids vector with flood depths at each location.

Best Answer

Related Solutions

[GIS] How to extract all distinct values from a PostGIS Raster

[GIS] Receiving NULL values from Point Sampling Tool in QGIS

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