Lambert projection with one or two standard parallels and proj4

Question

I'm sure there is a gap in my understanding, but I haven't been able to identify where it is, nor how to fill it. My issue is this:

• I'm transforming lon lat coordinates into a lambert conformal conic projection.

• The lambert projection uses a single standard parallel 46.244 and the central meridian is 17.0. And it's on a spherical ellipsoid, but that's not particularly relevant here. I am using packages that use the proj4 library.

• I test the transformation using linux tool cs2cs as follows:

  echo "15.220483 48.437298" | cs2cs +proj=lonlat +to +proj=lcc
  +lat_1=46.244 +lat_0=46.244 +lon_0=17. +a=6367470 +b=6367470 +ellps=sphere +datum=WGS84

  -131346.80 245318.16

• I also understand that the one-standard parallel form is the same as the two-standard parallel form if the two standard parallels are the same. It is more convenient for me to give all parallels in my scripts. So I also test with:

  echo "15.220483 48.437298" | cs2cs +proj=lonlat +to +proj=lcc +lat_1=46.244 +lat_2=46.244 +lat_0=46.244 +lon_0=17. +a=6367470 +b=6367470 +ellps=sphere +datum=WGS84

  -131290.66 245281.34

yet I get different results. Any ideas as to why?

And at very least, which should I use?

Answer 1

A good point made by Rene W.

Here is the answer:

add +no_defs to your first example in order to avoid unwanted default values

about this part: +a=6367470 +b=6367470 +ellps=sphere +datum=WGS84 +datum=WGS84 is a shorthand for: +ellps=WGS84 +towgs84=0,0,0.

You already have +ellps=sphere. Your reference surface cannot be a shpere and the WGS84 ellipsoid at the same time. As you already have +a=6367470 and +b=6367470, you should drop both +ellps=sphere and +datum=WGS84`

in case of doubts, use cs2cs -v for more info about which parameters are actually takes, and which are ignored

Hermann

and

The default values of lat_1 and lat_2 are 33 and 45, so in your first example you had +lat_1=46.244 +lat_2=45 which is not what you intended. The second example does what you wanted.

Charles

Answer 2

The answer is here!

many thanks to the responders on the proj mailing list.