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--- georef [2018/10/11 03:11] – [Coordinate Transformation] allchin09
+++ georef [2018/10/14 23:36] – [Coordinate Transformation] allchin09
@@ Line 76: / Line 76: @@
 A paper on the [[http://www2.unb.ca/gge/Personnel/Santos/Santos_SNB_Report_Grid_Shift-NAD27-NAD83.pdf|Generation of a NAD27-NAD83(CSRS) NTv2-type Grid Shift File for New Brunswick]] is also of interest.
+== An example - ANG to GDA94 ==
+The trial version of [[http://www.killetsoft.de/p_svpa_e.htm|SEVENPAR]] by Killet Software was used to produce local parameters for a 7 parameter Helmann transformation. The trial version is limited to only 25 common points (locations known in both source and target CRS) so ones near Sydney sourced from the NGRS were used.
+It's important to note the convention used (either Coordinate-Frame Rotation, or Position Vector) to define the three rotation parameters (signs are flipped between them).
+The output in position vector format was:
+^Parameter^Value^Units^
+|dx|195.056526|m|
+|dy|-112.959677|m|
+|dz|-172.456394|m|
+|rx|3.59354054|rad, seconds|
+|ry|5.39693644|rad, seconds|
+|rz|-1.34857381|rad, seconds|
+|s|-3.2756552|ppm|
+Combining these with the known values of the ellipsoid parameters of the ANS, a custom CRS can be defined in PROJ to allow transform between ANG and GDA94 coordinates. This can then be used in QGIS to 'reproject' ANG georeferenced rasters into a GDA94 CRS.
+The following PROJ string was used to define the ANG CRS:
+''+proj=longlat +a=6378339.78 +rf=294.26 +towgs84=195.056526,-112.959677,-172.456394,3.59354054,5.39693644,-1.34857381,-3.2756552 +no_defs''
+This CRS is then available to be applied to any raster.
 ==== Coordinate Conversion ====