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I am a complete n00b when it comes to GIS, so please bear with me. I started out by posting here: http://stackoverflow.com/questions/6963991/how-can-i-transform-the-coordinates-of-a-shapefile .

In short, I found a shapefile here, and I want to be able to convert that to Latitude/Longitude (Geodetic?) coordinates that I can store in my PostGIS database. I am having trouble working with the Ruby libraries, but that is a separate issue. First, I need to correctly understand the projection.

In the linked zip file, there is a .prj file, which contains the following (It is only one line, but I broke it up for readability):


I was unable to find tools that translate .prj files into a projection string, so I used this as a reference and came up with:

+proj=lcc +datum=NAD83 +ellps=GRS80 +lat_1=37.06666666666667 +lat_2=38.43333333333333 +lat_0=36.5 +lon_0=-120.5 +x_0=6561666.666666666 +y_0=1640416.666666667 +units=us-ft +pm=greenwich +no_defs 

I happen to know that the points in this shapefile should correspond to locations in San Francisco, so I could test my work:

cs2cs -f "%.8f" +proj=lcc +datum=NAD83 +ellps=GRS80 +lat_1=37.06666666666667 +lat_2=38.43333333333333 +lat_0=36.5 +lon_0=-120.5 +x_0=6561666.666666666 +y_0=1640416.666666667 +units=us-ft +pm=greenwich +no_defs +to +proj=lonlat +datum=WGS84 +ellps=WGS84
6011287.4999795845 2100857.2499904726
-164.18117482   17.72087079 0.00000000

That's about 1000km west of Hawaii, so clearly that's not right. I tried many different things, but the closest I got was by stripping the +units=us-ft off.

cs2cs -f "%.8f" +proj=lcc +datum=NAD83 +ellps=GRS80 +lat_1=37.06666666666667 +lat_2=38.43333333333333 +lat_0=36.5 +lon_0=-120.5 +x_0=6561666.666666666 +y_0=1640416.666666667 +pm=greenwich +no_defs +to +proj=lonlat +datum=WGS84 +ellps=WGS84
6011287.4999795845 2100857.2499904726
-126.98864867   40.47509388 0.00000000

That's about 500 km NW of San Francisco.

What am I missing here? Please educate this desperate newbie.
Worth noting: If I import the shapefile into the online ArcGIS Explorer, it understands it perfectly.

Thanks in advance.

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3 Answers 3

up vote 3 down vote accepted

I believe that the correct string for this projection is:

+proj=lcc +lat_1=37.06666666666667 +lat_2=38.43333333333333 +lat_0=36.5 +lon_0=-120.5 +x_0=2000000 +y_0=500000.0000000002 +ellps=GRS80 +datum=NAD83 +to_meter=0.3048006096012192 +no_defs

See: http://spatialreference.org/ref/esri/102643/

Regards, Nick.

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Just to clarify, the proj4 addendum ftp.remotesensing.org/proj/proj.4.3.pdf (p. 7) explains that the + parameters always need to be specified in metres, regardless of the units the projection is in. That's why the +x and +y definitions in Nick's answer are different, because Larry's parameters were in us-ft. This is something that has caught me out in the past. –  MerseyViking Aug 10 '11 at 9:28
Yes! This seemed to work. Thank you SO much. And thanks for the explanation MerseyViking. –  Larry Aug 10 '11 at 15:55

Coordinate reference systems and transformations are a complicated matter and are a source of trouble in practice.

I'm not aware of any tool that converts WKT to proj4. As a general hint, look up the PROJCS keyworks on the Spatial Reference website (that's how nhopton found his answer) to find hints to your projection.

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While there isn't a GDAL tool as such, it is four lines of Python to convert from WKT to Proj4. Posting code in a comment isn't easy, but you just need to create a osr.SpatialReference object, call ImportFromWkt() passing in the WKT string, and then call ExportToProj4() and out pops the answer. In this case, the PROJECTION parameter needs to be changed from Lambert_Conformal_Conic to Lambert_Conformal_Conic_2SP, but that is a corner case in an otherwise very robust implementation. –  MerseyViking Aug 10 '11 at 13:56
Great! Post it as a separate answer! –  EPSG31468 Aug 10 '11 at 15:36
I know I'm probably pushing my luck here, but if anybody has used RGeo for Ruby, please take a look at stackoverflow.com/questions/7006581/… –  Larry Aug 10 '11 at 15:57

As requested by EPSG31468, here's a very simple Python script to convert from any projection format handled by GDAL to Proj4 strings:

# This code is released into the public domain, but may be subject to the licence(s) of the third-party libraries that have been used.
# There is absolutely no warranty or guarantee of fitness for purpose.

import sys
import osgeo.osr as osr

projectionString = sys.stdin.read()

sr = osr.SpatialReference()
proj4 = sr.ExportToProj4()

if proj4 == "":
    print("Unable to convert input string.")

There's not a great deal of error handling, but it works with the strings I've tried it with.

To use it, type something like:

$ cat larrysWKT.txt | toproj4.py
+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 +units=us-ft +no_defs

or even:

$ echo EPSG:27700 | toproj4.py
+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs

Equivalently, Windows users will need to do this:

c:\> type larrysWKT.txt | python toproj4.py
c:\> echo EPSG:27700 | python toproj4.py

Naturally you'll need Python installed, as well as GDAL/OGR and the corresponding Python bindings. Linux users should be able to get this from their package repositories, and I think Windows users can install it from the OSGeo4W installer.

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