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I know how to define a CRS, but this particular one is giving me a headache. I was unable to find the EPSG code at spatial reference.

The main reason for asking this is that raster and vector data available from INEGI (Mexico) comes in ITRF 92. Also new cartography from the same source comes in ITRF 2008.

When I load a raster or a vector layer from INEGI their CRS properties only said "unknown datum based on GRS80 ellipsoid".

INEGI says that ITRF and WGS84 are "equivalent" to make maps in scales smaller than 1:50 000. Nevertheless I usually make maps in scales greater than 1:50 000

Any ideas are welcome, Thanks beforehand.

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Could you post links to the data, the actual files? Nick. –  nhopton Jun 26 '12 at 19:32
    
MerseyViking Then, I can not define ITRFxx datum in proj4? If so, do you recommend treating the data in vector or raster as WGS84? Nhopton Here you can download some river data in shp format inegi.org.mx/geo/contenidos/recnat/hidrologia/… Look particularly at the "Marco de Referencia" Link, here explains in detail the CRS information (in spanish though) Here you can download a DEM inegi.org.mx/geo/contenidos/datosrelieve/continental/… –  user8447 Jun 27 '12 at 14:38
    
Gerardo, please do not use answers to improve your question or respond to comments! You may edit the question or provide comments of your own. –  whuber Jun 27 '12 at 14:50
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3 Answers 3

I'm not sure you've looked very hard! Googling for ITRF92 gives me this as the top hit.

The code parameter shows 4914 which, according to spatialreference.org is the EPSG code. Looking at the WKT for it, it looks like it matches the georepository.com data:

GEOCCS["ITRF92",
DATUM["International Terrestrial Reference Frame 1992",
    SPHEROID["GRS 1980",6378137.0,298.257222101,
        AUTHORITY["EPSG","7019"]],
    AUTHORITY["EPSG","6651"]],
PRIMEM["Greenwich",0.0,
    AUTHORITY["EPSG","8901"]],
UNIT["m",1.0],
AXIS["Geocentric X",OTHER],
AXIS["Geocentric Y",EAST],
AXIS["Geocentric Z",NORTH],
AUTHORITY["EPSG","4914"]]

ITRF2008 is a little trickier, I grant you. Normally I'd go to EPSG's own catalogue, but it seems to be down at the moment.

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thanks for the answer! I can recognize as a prj file in spatial reference, nevertheless the spirit of my question is how to define the CRS in proj4 format, in order that I can work with it in Qgis. Data I download (raster or vector) from INEGI does have such prj, but as I wrote before, I only have the "unknown datum based on GRS80 ellipsoid"... –  user8447 Jun 26 '12 at 17:15
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Ah! There's your problem, Proj4 can't use geocentric coordinates; it's strictly planar, which is why @iant's link yields an empty page. –  MerseyViking Jun 27 '12 at 9:05
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Perhaps it would be worth saving a couple of your ITRF shapefiles as KML to see how they plot on the ground in Google Earth. I've tried this with one of the hydrology layers containing some coastline and it looks pretty good to me, see the picture. You'll see the scale bar represents 679 metres on the ground.

Perhaps the difference between ITRF and WGS84 is analogous to the difference between ETRS89 and WGS84, which is about 0.5 m at present (here in England). Nick.

enter image description here

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QGIS Master now has EPSG:4914 incorporated with the following proj string:

+proj=geocent +ellps=GRS80 +units=m +no_defs

In addition, it might be necessary to obey datum shifts due to tectonic movement of the earth surface. This could be expreessed by adding a regional +towgs84 parameter.

But:

The data linked above does not come in earth-centred coordinates, but in degrees. So I would start of with EPSG:4326 and look if it fits; then add +towgs84 parametres to get the tectonic shift if necessary.

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