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| author | Frank Warmerdam <warmerdam@pobox.com> | 2002-06-13 14:24:43 +0000 |
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| committer | Frank Warmerdam <warmerdam@pobox.com> | 2002-06-13 14:24:43 +0000 |
| commit | 1b398463f8a37f6d79ab537246cf37a4083a3264 (patch) | |
| tree | e6c0370709aad8575da9f68084b86e327d3176c1 /html | |
| parent | 41fc20bf92d40c75c94f762813a9a4879a7a4ce7 (diff) | |
| download | PROJ-1b398463f8a37f6d79ab537246cf37a4083a3264.tar.gz PROJ-1b398463f8a37f6d79ab537246cf37a4083a3264.zip | |
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git-svn-id: http://svn.osgeo.org/metacrs/proj/trunk@1021 4e78687f-474d-0410-85f9-8d5e500ac6b2
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diff --git a/html/gen_parms.html b/html/gen_parms.html new file mode 100644 index 00000000..95e38983 --- /dev/null +++ b/html/gen_parms.html @@ -0,0 +1,85 @@ +<html> +<head> +<title>PROJ.4 - General Parameters</title> +</head> +<body BGCOLOR="#FFFFFF"> +<h1>PROJ.4 - General Parameters</h1> + +This document attempts to describe a variety of the PROJ.4 parameters +which can be applied to all, or many coordinate system definitions. This +document does not attempt to describe the parameters particular to particular +projection types. Some of these can be found in the GeoTIFF +<a href="http://www.remotesensing.org/geotiff/proj_list/">Projections +Transform List</a>. The definitative documentation for most parameters +is Gerald's original documentation available from the main PROJ.4 page. <p> + +<h2><a name="towgs84">towgs84 - Datum transformation to WGS84</a></h2> + +Datum shifts can be approximated by 3 parameter spatial translations (in +geocentric space), or 7 parameter shifts (translation + rotation + scaling). +The parameters to describe this can be described using the <b>towgs84</b> +parameter.<p> + +In the three parameter case, the three arguments are the translations to the +geocentric location in meters.<p> + +For instance, the following demonstrates converting from the Greek GGRS87 +datum to WGS84.<p> + +<pre> +% cs2cs +proj=latlong +ellps=GRS80 +towgs84=-199.87,74.79,246.62 \ + +to +proj=latlong +datum=WGS84 +20 35 +20d0'5.467"E 35d0'9.575"N 8.570 +</pre> + +The EPSG database provides this example for transforming from WGS72 to WGS84 +using an approximated 7 parameter transformation.<p> +<pre> +% cs2cs +proj=latlong +ellps=WGS72 +towgs84=0,0,4.5,0,0,0.554,0.219 \ + +to +proj=latlong +datum=WGS84 +4 55 +4d0'0.554"E 55d0'0.09"N 3.223 +</pre> + +The seven parameter case uses <i>delta_x</i>, <i>delta_y</i>, <i>delta_z</i>, +<i>Rx - rotation X</i>, <i>Ry - rotation Y</i>, <i>Rz - rotation Z</i>, +<i>M_BF - Scaling</i>. The three translation parameters are in meters as +in the three parameter case. The rotational parameters are not in physical +units. They are something like the sine of the rotational angle times the +ellipoid axis length, but I don't know the exact details of how to derive +these from a physical description of the rotation. What I do know is that +they match the parameters used for transformation method 9606 in the EPSG +database. The scaling is apparently the scale change in parts per million.<p> + +A more complete discussion of the 3 and 7 parameter transformations can be +found in the EPSG database (trf_method's 9603 and 9606). Within PROJ.4 +the following calculations are used to apply the <b>towgs84</b> transformation +(going to WGS84). The x, y and z coordinates are in geocentric coordinates. + +Three parameter transformation (simple offsets): + +<pre> + x[io] = x[io] + defn->datum_params[0]; + y[io] = y[io] + defn->datum_params[1]; + z[io] = z[io] + defn->datum_params[2]; +</pre> + +Seven parameter transformation (translation, rotation and scaling): + +<pre> + #define Dx_BF (defn->datum_params[0]) + #define Dy_BF (defn->datum_params[1]) + #define Dz_BF (defn->datum_params[2]) + #define Rx_BF (defn->datum_params[3]) + #define Ry_BF (defn->datum_params[4]) + #define Rz_BF (defn->datum_params[5]) + #define M_BF (defn->datum_params[6]) + + x_out = M_BF*( x[io] - Rz_BF*y[io] + Ry_BF*z[io]) + Dx_BF; + y_out = M_BF*( Rz_BF*x[io] + y[io] - Rx_BF*z[io]) + Dy_BF; + z_out = M_BF*(-Ry_BF*x[io] + Rx_BF*y[io] + z[io]) + Dz_BF; +</pre> + +</body> +</html> |