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I am trying to create a map with a layer of county tax assessor property parcels plus NED elevation data.

The projection of the tax assessor layer is a weirdo -- looks like user-created by the county. I got the NED layer to change to that weirdo projection, or so I thought.

But when I go to intersect the NED layer with the tax assessor layer, I always get the "CRS of the layers did not match" warning, and the resultant new layer is empty.

I've tried using GRASS to re-project the NED layer, but cannot figure out how to do it that way either.

What should I do? I just want to use the parcels layer to intersect (like clip) the NED layer.


Update:

The tax assessor parcels layer gives this as its projection:

+proj=lcc +lat_1=37.06666666666667 +lat_2=38.43333333333333 +lat_0=36.5 +lon_0=-120.5 +x_0=2000000 +y_0=500000.0000000001 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs

In the "Project Properties" window, it is listed under the heading "User Defined Coordinate Systems". What is it? And how can I get it to jive with the USGS NED data (which is, I think, unprojected and based on datum NAD83)?

Is "projection" in this QGIS usage the same as CRS? Thank you.

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2 Answers 2

It's probably a state plane (hopefully the state you are in). But you don't need to really know what the projection is to use it. All you need to do is reproject this shapefile to a "well known projection" (preferably the one your other data is in). For that you'll want ogr2ogr

ogr2ogr -s_srs "+proj=lcc +lat_1=37.06666666666667 +lat_2=38.43333333333333 +lat_0=36.5 +lon_0=-120.5 +x_0=2000000 +y_0=500000.0000000001 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" -t_srs epsg:4326 src.shp dest.shp

Change 4326 to whatever projection the NED data is in.

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I really appreciate how you guys help a new guy! Thanks. As above, I found out that the NED elevation data are unprojected but are referenced to datum NAD83. Can I reproject the tax assessor data to an unprojected status in NAD83? Thanks again. –  Catlike Mar 30 '12 at 21:58
1  
It's 2227, NAD83 State Plane California zone 3 (III) in US survey ft. The NED data is using 4269, NAD83. I found the CRS in the EPSG registry, epsg-registry.org, by knowing the lat/lon. It's always useful to say where data is located. –  Melita Kennedy Mar 31 '12 at 2:26
    
Melita Kennedy -- You are most helpful! Thank you kindly. –  Catlike Mar 31 '12 at 5:05
    
I had a similar question and the answer came down to a slight difference between ESRI's definition of the state plane I was using and EPSG's definition. Andre Joost provided a great answer and calculated the difference between the two to be 0.009 mm. gis.stackexchange.com/questions/43378/… –  HeyOverThere Jan 24 '13 at 15:24

Both layers have to overlay without on-the-fly reprojection turned on if you want to perform an intersection.

Try to reproject the tax assessor layer to the projection of the NED data. You can do that on the command line like @iant described or use "Save as ..." dialog in QGIS where you can also pick a target CRS.

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Thank you! Now, according to the USGS NED website, "The NED data are not projected, but are provided in geographic coordinates (latitude and longitude) in units of decimal degrees, horizontally referenced to the North American Datum of 1983". So can I reproject the tax assessor layer to an unprojected state in NAD83? –  Catlike Mar 30 '12 at 21:57
    
"Unprojected" in terms of USGS means in degrees. You can use EPSG:4269 NAD83 for that. –  Andre Joost Feb 14 at 6:43

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