specific development discussions on the Cartographic Projections Library (PROJ.4)

Charles Karney | 15 May 2013 23:08

Improvements to geod and invgeod

geod and invgeod, programs for performing geodesic calculations, have
been part of proj.4 since at least 1995.  There are several problems
with them:

* the overall accuracy is limited;

* the results of inverse calculations are sometimes wildly wrong;

* there's no documented API for the geodesic capabilities;

* there's no provision from computing various quantities associated with
   geodesics (in particular the area of a geodesic polygon).

These problems will be addressed in the next release of proj.4 with the
inclusion of the algorithms for geodesics from GeographicLib.  These are
documented in

   C. F. F. Karney,
   Algorithms for geodesics,
   J. Geodesy 87(1), 43-55 (Jan. 2013)
   http://dx.doi.org/10.1007/s00190-012-0578-z
   Addenda: http://geographiclib.sf.net/geod-addenda.html

Points to note:

* The changes have been checked into subversion but are not part of a
   release yet.  To try them out, check out proj.4 from subversion with

     svn checkout http://svn.osgeo.org/metacrs/proj/trunk/proj

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Ed Campbell | 2 May 2013 20:37

Robinson projection coefficients

A trac ticket on the Robinson projection has recently been reopened
(http://trac.osgeo.org/proj/ticket/113). The issue is that an applied
correction to the constants that define the Robinson projection was
only a partial fix. The higher-order interpolation coefficients also
needed to be updated. Below is a short python script that I have
written to determine the coefficients:

#!/usr/bin/env python2.7
"""
Short script to calculate the interpolation coefficients
used by proj.4 Robinson projection.

.. note::

    The scipy interpolate method is not a natural cubic
    spline (curvature zero at the endpoints).

"""
import sys

import numpy
import scipy.interpolate

def spline_coeffs(x, y):
    """
    Calculates the 5 deg interpolation coefficients based on
    input arrays x and y where y = f(x).

    """
    s = scipy.interpolate.InterpolatedUnivariateSpline(x, y)

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Linton Corbet | 25 Apr 2013 09:51

Lambert projection with one or two standard parallels and proj4

Hello all,

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 sphere, 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 - let proj do the work. 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, what am I doing wrong and which approach should I use?

Kind regards,
Linton Corbet

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Phil Elson | 22 Apr 2013 17:23

Programmatically obtaining the domain of a projection

I've got a case where I would like to identify the domain of a projection programmatically. Take the geos projection for example:

$> invproj +proj=geos +h=35785831 +units=m
5.3e6 0
68d48'11.267"E    0dN

5.4e6 0
74d56'57.772"E    0dN

5.5e6 0
*    *

$> invproj +proj=geos +h=50000000 +units=m
5.5e6 0
69d42'46.646"E    0dN

5.6e6
74d40'19.703"E    0dN

5.7e6
*    *

Are the mathematical limits of Proj4 projections generally available (the C interface is fine), or is using a numerical method the only way to calculate the domain?

Notes:
* There are some domains/bounds available on spatialreference.org (e.g., http://spatialreference.org/ref/epsg/4277/) but they are not exhaustive, are transformed to WGS84 and have presumably been calculated manually.

* I've had a quick look at PJ_geos to see if this particular example has a simple solution (and indeed grepped for "limits", "bounds", and "range" in the whole proj.4 source) without much luck.

* I've already got a programmatic solution to this, I'm really looking to find out if there is a more elegant mathematical answer to my problem.

Many thanks in advance,

Phil

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Mikael Rittri | 16 Apr 2013 11:46

Calculate map resolution / LCC

Hello Achim.

I would calculate the resolution like this:

Take the two calibration points that are farthest away;
let’s call them P and Q.

1) Compute the true distance in meters between them:

a) Either take the long/lat coordinates for P and Q,

and use for example http://geographiclib.sourceforge.net/cgi-bin/Geod

b) Or else, take the projected coordinates (if you have them),

and just use the theorem of Pythagoras to calculate the
distance in meters. (This assumes that the local scale factor of the

projection is near 1 in the area of interest, which it seems to be.)

2) Compute the image distance between P and Q, in pixels.

Just use the theorem of Pythagoras on the pixel coordinates.

If the real-world distance between P and Q is s meters,
while the pixel distance is p pixels, then the image resolution

is

s / p

meters per pixel.

Best regards,

Mikael Rittri

Carmenta

Sweden

http://www.carmenta.com

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Veronica Andreo | 11 Apr 2013 09:41

Re: Proj Digest, Vol 107, Issue 2

Thanks a lot for the information, Clifford! I'll write Ruben wright now!

Best,

Veronica

2013/4/10 <proj-request <at> lists.maptools.org>

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Today's Topics:

1. metric projection to map all Argentina (Veronica Andreo)
2. Re: metric projection to map all Argentina (Clifford J Mugnier)

----------------------------------------------------------------------

Message: 1
Date: Wed, 10 Apr 2013 09:58:00 +0200
From: Veronica Andreo <veroandreo <at> gmail.com>
Subject: [Proj] metric projection to map all Argentina
To: proj <at> lists.maptools.org
Message-ID:
<CAAMki4GA+8t=yrE+LmjYjMYgaFSdWv3KTjNYUGf_vDoWBBufvA <at> mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"

Hello,

I'm working with different raster and vector data for all Argentina and I
can not find the official metric projection to represent the whole country
at once... The official projection according to the National Geography
Institute is Gauss-Kruger, but I think that is only useful for smaller
areas... Does anyone know what is normally used or have any suggestion on
what to use?

Thanks a lot in advance!

Veronica Andreo
Argentina
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Message: 2
Date: Wed, 10 Apr 2013 13:29:56 +0000
From: Clifford J Mugnier <cjmce <at> lsu.edu>
Subject: Re: [Proj] metric projection to map all Argentina
To: PROJ.4 and general Projections Discussions
<proj <at> lists.maptools.org>
Message-ID:
<0DA58EB5D84A2744B1229C32174A8D526BBEFBB2 <at> BY2PRD0610MB352.namprd06.prod.outlook.com>

Content-Type: text/plain; charset="iso-8859-1"

The international authority for all things geodetic/cartographic in Argentina is Mr. Rub?n Rodr?guez [rubenro <at> fibertel.com.ar]. If there is a national standard, Rub?n knows it.

Clifford J. Mugnier, c.p., c.m.s.

Chief of Geodesy

LSU Center for GeoInformatics (ERAD 266)

Dept. of Civil Engineering (P.F. Taylor 3531)

LOUISIANA STATE UNIVERSITY

Baton Rouge, LA 70803

Academic: (225) 578-8536

Research: (225) 578-4578

Cell: (225) 328-8975

honorary lifetime member, lsps

fellow emeritus, asprs

member, apsg

________________________________
From: proj-bounces <at> lists.maptools.org [proj-bounces <at> lists.maptools.org] on behalf of Veronica Andreo [veroandreo <at> gmail.com]
Sent: Wednesday, April 10, 2013 2:58 AM
To: proj <at> lists.maptools.org
Subject: [Proj] metric projection to map all Argentina

Hello,

I'm working with different raster and vector data for all Argentina and I can not find the official metric projection to represent the whole country at once... The official projection according to the National Geography Institute is Gauss-Kruger, but I think that is only useful for smaller areas... Does anyone know what is normally used or have any suggestion on what to use?

Thanks a lot in advance!

Veronica Andreo
Argentina
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End of Proj Digest, Vol 107, Issue 2
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Veronica Andreo | 10 Apr 2013 09:58

metric projection to map all Argentina

Hello,

I'm working with different raster and vector data for all Argentina and I can not find the official metric projection to represent the whole country at once... The official projection according to the National Geography Institute is Gauss-Kruger, but I think that is only useful for smaller areas... Does anyone know what is normally used or have any suggestion on what to use?

Thanks a lot in advance!

Veronica Andreo

Argentina

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Swagath Navin | 11 Mar 2013 13:46

coordinates in decimal degrees and Albers Equal Conic projection

Dear all,

Sorry for being naive, i am completely new to map projections. I have a 
data set with coordinates in decimal degrees (-14.36, 53.23) with the 
following information:

"The coordinates are designed to be used at the scale of the European 
continent (Albers Equal Area Conic projection, with the following 
projection settings: Scale = 1; False Easting(m) = 5000000; False 
Northing(m) = 0; Longitude Origin = 20; Latitude Origin = 0; Standard 
Parallel 1 = 45; Standard Parallel 2 = 55, and the Ellipsoid is WGS 84)"

i would like to know if the coordinates are already in WGS84 or is there 
any conversion required.

Thanks a lot for your time and help.

Navin
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John Lilley | 26 Feb 2013 00:25

[proj4] beginner questions

Greetings,

We are getting started with proj4 and I am after some fairly basic things to being, I was hoping you could help me get started. Our basic need is:

* Reading the datum specification from ShapeFiles, TAB, MID/MIF.

* Converting an arbitrary datum to WGS84 for internal processing.

* Presenting user with a pick list of datum for output.

* Converting from WGS84 back to user-chosen datum.

I suppose it is never that simple, but if you can point me in the right direction, that would be much appreciated.

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Carl Godkin | 19 Feb 2013 04:29

EPSG codes for regional maps..

Hi,

EPSG codes provide a very concise representation for coordinate systems but I don't know how to look up EPSG-defined coordinate systems for large regional maps such as North America or Europe. Are there any?

The only one I've found for North America is the US National Atlas Equal Area which is 2163. But it's based on an unknown datum so it's not always very useful. I also know about 3857 (aka 900913) which is a pseudo Mercator projection of the whole world

Are there others in the EPSG database for continent-scale maps? How do I search for them?

Thanks very much,

carl

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Domenico Nappo | 15 Feb 2013 20:39

proj4 strings from lat lon pcraster maps

Hi folks,
I'm quite newbie in proj4 (and geo world in general) so sorry if it seems a dummy question (I was sure to find a faq on this but maybe I miss something crucial here).

I'm using some geo interpolation libraries (python) which require proj4 strings and area extents for interpolation methods.
As target grids, I got fixed pcraster maps, representing (I suppose...) regular gaussian grids as 2 maps (lats and lons).

How to define proj4 strings out of this information?

Thanks in advance!

--
d

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