[GIS] Get pixel value from a raster in a xy table

arcgis-10.1asciiexcelqgisraster

I'm trying to find a way to convert a raster (in my case it's a Landsat satellite image) into a table with 3 columns which would represent: 1) x coordinates (the so called "xllcorners" in ASCII terms); 2) y coordinates (the so called "yllcorners"); 3) the relative pixel values.
As I'm working with 8-bit pixel-depth raster, my table should look like this at the end:

   X       Y      Pixel
493185  4989885     0
493185  4989915    127
   .       .        .
   .       .        .
   .       .        .

I tried to convert the raster into ASCII format and it gave me a grid with 7911 columns and 7211 rows. No problem so far, but since I want to organize the entire grid as it fits my 3 columns requirement, I haven't been able to import it to excel (where I'd like to do my calculations) properely (namely, it seems replacing and cutting off the spaces, inserting commas and so on didn't work so fine as there's always something different than in the "txt" ASCII grid, and it's not a squared grid neither, which complicates the things).
I'm using both ArcGIS 10.1 and QGIS 1.8 if it could be of interest.

Best Answer

GRASS GIS r.out.xyz tool

You can use the r.out.xyz tool in the GRASS toolbox in QGIS. The function exports a raster map as a list of x,y,z values into an ASCII text file, skipping x,y coordinates for raster cells containing a NULL value. For more information, see the r.out.xyz help file. The disadvantage is that you need first to create a GRASS database and import (or register) the raster layers.

GDAL gdal2xyz tool

Alternatively, you can use the gdal tool gdal2xyz.py, which is a command line tool that does basically the same as r.out.xyz. It should be included with the GDAL that comes with QGIS. See this similar question on gis.stackexchange, which tells you to run it from an OSGeo4W command line window as:

gdal2xyz -csv infile.asc outfile.csv

Related Solutions

Convert Lat Lon Value to Raster File Using R – Data Transformation Guide

Several steps required:

You say it's a regular 1km grid, but that means the lat-long aren't regular. First you need to transform it to a regular grid coordinate system so the X and Y values are regularly spaced.

a. Read it into R as a data frame, with columns x, y, yield.
```
pts = read.table("file.csv",......)
```
b. Convert the data frame to a SpatialPointsDataFrame using the sp package and something like:
```
library(sp)
library(rgdal)
coordinates(pts)=~x+y
```
c. Convert to your regular km system by first telling it what CRS it is, and then spTransform to the destination.
```
proj4string(pts)=CRS("+init=epsg:4326") # set it to lat-long
pts = spTransform(pts,CRS("insert your proj4 string here"))
```
d. Tell R that this is gridded:
```
gridded(pts) = TRUE
```
At this point you'll get an error if your coordinates don't lie on a nice regular grid.

Now use the raster package to convert to a raster and set its CRS:

r = raster(pts)
projection(r) = CRS("insert your proj4 string here")

Now have a look:
```
plot(r)
```
Now write it as a geoTIFF file using the raster package:
```
writeRaster(r,"pts.tif")
```

This geoTIFF should be readable in all major GIS packages. The obvious missing piece here is the proj4 string to convert to: this will probably be some kind of UTM reference system. Hard to tell without some more data...

[GIS] Z-value missing in attribute table of shapefile from ASCII 3D to Feature Class

As FelixIP said in the comments, the Z values should be in there: the "Z" in the Shape type of "Multipoint ZM" indicates that your layer is z-enabled, so if you did the conversion from .XYZ to Shapefile correctly, your Z values are in there. It's just that Z values don't show up in a feature's attribute table by default, you need to add them manually. You can use the 3D Analyst Add Z Information tool to do that if you want to check your data, but any of the ESRI tools which operate on Z values should work fine with your shapefile the way it is.

Best Answer

Related Solutions

Convert Lat Lon Value to Raster File Using R – Data Transformation Guide

[GIS] Z-value missing in attribute table of shapefile from ASCII 3D to Feature Class

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