[QGIS-trac] Re: [Quantum GIS] #1035: automated generation of srs.db (in line with postgis, proj and gdal)

[Quantum GIS]

#1035: automated generation of srs.db (in line with postgis, proj and gdal)
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        Reporter:  rduivenvoorde                    |         Owner:  nobody           
            Type:  enhancement                      |        Status:  new              
        Priority:  minor: annoyance or enhancement  |     Milestone:                   
       Component:  Build/Install                    |       Version:  HEAD             
      Resolution:                                   |      Keywords:  srs.db projection
Platform_version:                                   |      Platform:  All              
        Must_fix:  No                               |   Status_info:  0                
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Comment (by hamish):

 Replying to [comment:7 jef]:
 > Good idea - but I just noticed that QGIS also does this if you
 > configure it to "Prompt for Projection" in Settings/Options in
 > the projection tab.  The default is to fallback to WGS84.

 No it is something different. That prompts you for the projection to use,
 letting you pick from the entries in QGIS's srs.db SQLite database. But
 the srs.db entries are currently missing data :)

 What I was speaking of with GRASS is a companion GUI window that, once you
 have selected an EPSG "projection" code, lets you pick the associated
 datum transform parameters to use with that. But "projection" is the wrong
 thing to call an EPSG code.....


 terminology I'm using, as I understand it:

 [PROJ.4 parameter in brackets]

  * projection [+proj=] - mathematical bending method to impose 2D from a
 3D world (e.g. tmerc [transv mercator], lcc [lambert conformal conic]).
 They are often centered at a specific lat/lon [+lat_0= +lon_0=]

  * datum [+datum=] - where the center of mass of the world is (where you
 place 0,0,0), typically defined with a standard ellipsoid (deformation of
 geoid from a perfect sphere) Datums created after the satellite age are
 typically very close. WGS84 defines both a datum and an ellipsoid with the
 same name, so that's slightly confusing. ?{If the software knows how it
 can expand +datum= into +ellps=,+a=,+k=. +a,+b are radius of Earth
 (major/minor?), +k is scaling factor.}?

  * false eastings & northings [+x_0= +y_0=] - number to add to the
 projected coordinates. often to keep them always positive or distinct from
 other systems, often in the millions

  * SRS [+init=epsg:1234] - spatial reference system (eg the EPSG codes) a
 commonly used combination of a projection, datum and/or ellipsoid, false
 eastings & northings, etc.  Strictly speaking the "projection" is just the
 +proj= part, not the whole SRS even though it is often called that. Not
 all SRSs define a datum, in those cases you just get the ellipsoid(?).

  * datum transform params [+towgs84=] - any given SRS ''may'' use slightly
 different coefficients to convert to wgs84 (as the middleman datum). often
 these will be given as 3 or 7 numbers which are tuned to an average for
 the entire country or to give very good results for a single province. So
 for a given SRS you get to choose which state or province you want it
 tuned for, or a generalized set of terms. There are also distortion grids
 (aka NTv2 or nadgrids) you can get which supply local corrections for the
 entire country. These will typically give better results than a 3 or 7
 term +towgs84= param, but they add a file dependency to the mix which adds
 installation difficulties.

 further reading:
  http://grass.osgeo.org/wiki/Gis_Concepts


 if you look in /usr/share/proj/epsg you will see these things, along with
 some like:
 {{{
 # Qornoq 1927 / Greenland zone 5 west
 # Unable to translate coordinate system EPSG:2304 into PROJ.4 format.
 }}}
 Perhaps in that one PROJ doesn't do polar sterographic?

 and some old ones like:
 {{{
 # Dabola 1981 / UTM zone 28N (deprecated)
 <2063> +proj=utm +zone=28 +a=6378249.2 +b=6356515
 +towgs84=-23,259,-9,0,0,0,0 +units=m +no_defs  <>
 }}}



 hope that helps, and that I've not butchered the concepts too badly.


 Hamish

-- 
Ticket URL: <http://trac.osgeo.org/qgis/ticket/1035#comment:8>
Quantum GIS <http://qgis.org>
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