I'm working with QGIS and trying to change the CRS of some maps. Is is possible to do it without the .prj file?
You have to take in to account somethings. INEGI (National Geoography Instutute of Mexico) has data in UTM, Lambert Conformal Conic (this one with special parameters)and some in lat lon unprojected. Also You can get the data in NAD27 or ITRF92 datums. As Curlew says, before doing any reprojection you must know the layer original crs. I don know why INEGI does not provide sometimes the prj file but stores the CRS information in a text file. Find the text file and check the CRS information.
Bear in mind also that if the layer is in NAD27 you have to know the parameters for the wgs84 transformation. According to NIMA (2000), Department of Defense, World Geodetic System 1984: Its Definition and Relationships with local Geodetic Sytems, appendix B 6.5 the parameters are:
If the layers is in ITRF92 datum (either UTM, LCC or latlon) You will not find a CRS based on ITRF92 datm in QGIS (UTM. LCC or latlon). INEGI suggest that if your goal is to produce a map in a scale smaller than 1:50,000 you can use a WGS84 datum.
If the layer is in LCC you have to make the INEGI LCC CRS and store it in QGIS as a User defined CRS. The parameters are:
+proj=lcc +lat_1=17.5 +lat_2=29.5 +lat_0=12 +lon_0=-102 +x_0=2500000 +y_0=0 +ellps=WGS84 +datum=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
Hope it helps
You can always set the CRS of a layer with rightclick on the layer ->
Set CRS for Layer. After that, you can reproject the data with
If you have this case of missing .prj file often, deactivate the CRS preset in
Settings -> Options -> CRS tab. there you can specify that QGIS should ask for a CRS if no .prj is present. In your configuration, the default may be set to WGS84 or to
Layer of the project, which gives you wrong data.