Raster – Can an ASCII Grid Contain CRS or Projection

arcmapcoordinate systemesri-ascii-rasterrraster

I recently discovered the raster library in R. With that library one can define a raster's coordinate reference system (crs) or projection. For example, to set the projection as NAD83 UTM 10N:

crs(raster) <- CRS(+init=EPSG:26910')

I wrote that raster to file and imported into ArcMap, but Arc said that the file was missing a spatial reference system.

In general, is it possible for an ASCII file to contain/define its CRS or projection? If so, can that be done with R's raster library or with ArcMap?

Best Answer

There's no provision for coordinate system information in the ESRI ASCII raster file format:

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/ESRI_ASCII_raster_format/009t0000000z000000/

You might get the coordinate system info in a metadata file accompanying an ESRII ASCII grid file, but there's no standard for it as far as I know.

Use another format whenever possible, such as geoTIFF. Most GIS will happily read geoTIFF files and set the coordinate system correctly from the embedded metadata.

Related Solutions

[GIS] How to get the Parameters for a Lambert conformal conic projection CRS in QGIS 2.6

In QGIS, goto Settings -> Custom CRS, and Copy from existing CRS.

take a lcc projection based on WGS84, like EPSG:3762

+proj=lcc +lat_1=-54 +lat_2=-54.75 +lat_0=-55 +lon_0=-37 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs

and replace the values to your system:

+proj=lcc +lat_1=17.5 +lat_2=19.5 +lat_0=12 +lon_0=-102 +x_0=2500000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

I took ITRF92 and WGS84 as identical. See this page for more information about that:

ftp://itrf.ensg.ign.fr/pub/itrf/WGS84.TXT

[GIS] ASCII to Raster causing projection and scale issues

Expanding on my previous comment...

ASCII files are a valid (supported) raster format by ArcGIS and QGIS, GDAL etc. you should be able to consume the raster directly or import using a tool like Copy Raster or in arcpy (python window):

Inp = arcpy.Raster(r'c:\your\path\to\Raster.asc')
Inp.save(r'c:\export\path\File.tif')

The Raster.save() method works out from the extension what driver to use so any ArcGIS writable raster format that supports the pixel depth is fine.

Before you do anything with the file check that it's actually an Esri ASCII and not some other ASCII file format, open one of your files in your favorite text editor and view the top few lines; an Esri ASCII file has the header:

NCOLS xxx
NROWS xxx
XLLCORNER xxx
YLLCORNER xxx
CELLSIZE xxx
NODATA_VALUE xxx
row 1

I have also encountered XYZ data being called ASCII (well, they are ASCII too, that term just means 'text').. even GDAL is fussy about XYZ, the records must be gridded (regular); LiDAR XYZ (irregular) cannot be converted directly by GDAL or ArcGIS.

Which makes you wonder why there even is a 'ASCII to Raster' tool; it's a throwback to a time when Esri ASCII wasn't supported directly by Esri but rather an interchange format (yes, the 90's) - when Esri GRID was the main spatial raster format but GRIDs couldn't be written to tape unless you copied the whole workspace so to send a raster dataset on 8mm Exabyte Mammoth-2 tape for example it needed to be exported to a non-workspace raster that supported georeference and the only viable format that supported 32bit float was ASCII text format.

Since then GeoTIFF (and CD drives) have been invented so IMHO Esri ASCII raster format should be retired with extreme prejudice as a seriously bloated interchange format that has no real benefits over GeoTIFF, except for zipping very well, and yet I find myself exporting them to meet client requirements still...

Best Answer

Related Solutions

[GIS] How to get the Parameters for a Lambert conformal conic projection CRS in QGIS 2.6

[GIS] ASCII to Raster causing projection and scale issues

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