georef
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georef [2018/10/10 23:28] – [Coordinate Conversion] allchin09 | georef [2021/03/11 00:31] (current) – [Coordinate Conversion] allchin09 | ||
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+ | === NTv2 Transformation Grids === | ||
+ | |||
+ | Another way to transform coordinates is through a transformation grid. This is essentially a fancy version of a 'block shift' where the change (dLat and dLong or dE and dN) between the two CRSs was specified over a large region. An example of a rough block shift is adding 100m to eastings, and 200m to northings when transforming between AMG66 and MGA94. | ||
+ | |||
+ | A transformation grid uses the same principle but defines the shift at regular intervals throughout a region (eg, different values every minute). The NTv2 transformation grid is a specific file containing this information developed by Canada but now used throughout the world. | ||
+ | |||
+ | In many cases, you can find an NTv2 grid for the transformation you wish to perform. ICSM has produced multiple grids to transform between GDA94 and GDA202 which they have made [[https:// | ||
+ | |||
+ | If a grid file does not exist for the transformation you wish to perform, it is possible (but appears difficult) to create your own. Some information about process flows was discussed in 2013 on the [[http:// | ||
+ | |||
+ | The Victorian land agency also appear to have a free tool perform transformations called [[https:// | ||
+ | |||
+ | Killet Software has an [[http:// | ||
+ | |||
+ | A paper on the [[http:// | ||
+ | |||
+ | == An example - ANG to GDA94 == | ||
+ | |||
+ | The trial version of [[http:// | ||
+ | |||
+ | 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, | ||
+ | |ry|5.39693644|rad, | ||
+ | |rz|-1.34857381|rad, | ||
+ | |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 ' | ||
+ | |||
+ | The following PROJ string was used to define the ANG CRS: | ||
+ | |||
+ | '' | ||
+ | |||
+ | This CRS is then available to be applied to any raster. | ||
==== Coordinate Conversion ==== | ==== Coordinate Conversion ==== | ||
- | [[https://proj4.org/ | + | [[https://proj.org/ |
+ | |||
+ | Note that PROJ is being updated and dramatically changed - a useful overview [[https:// | ||
Handy tables of parameters can be found [[https:// | Handy tables of parameters can be found [[https:// | ||
- | ' | + | === Coordinate coversion for 1 inch to the mile millitary maps (ANG) === |
+ | |||
+ | |||
+ | ' | ||
^Parameter | ^Parameter | ||
Line 79: | Line 124: | ||
| false easting | | false easting | ||
| false northing | 800, | | false northing | 800, | ||
- | | false northing | 731,518.73| metres | + | | false northing | 731, |
| zones | 1 to 8 | | | | zones | 1 to 8 | | | ||
| zone width | 5 | degrees | | zone width | 5 | degrees | ||
Line 85: | Line 130: | ||
| zone latitude origin | 34S | degrees | | zone latitude origin | 34S | degrees | ||
| scale factor | | scale factor | ||
+ | |||
+ | |||
+ | On these maps, Sydney Observatory was used as a datum for triangulation, | ||
+ | ^Format ^Latitude | ||
+ | |DMS |S 33d 51m 41.10s | ||
+ | |DD |S 33.861417d | ||
+ | |||
+ | The [[http:// | ||
+ | ^Datum | ||
+ | |Clarke |S33°51' | ||
+ | |AGD66 | ||
+ | |AGD84 | ||
+ | |GDA94 | ||
Note: | Note: | ||
Line 105: | Line 163: | ||
Which returns: E=450, | Which returns: E=450, | ||
+ | An explaination of parameters is as follows: | ||
+ | === Parameters === | ||
+ | |||
+ | == +proj == | ||
+ | * Description: | ||
+ | * Value: tmerc | ||
+ | * Value description: | ||
+ | * [[https:// | ||
+ | * Projection used by ANG from June 1936 | ||
+ | * Note: prior to 1936, a polyconic projection was used | ||
+ | |||
+ | == +lat_0 == | ||
+ | * Description: | ||
+ | * Value: 34s | ||
+ | * Value description: | ||
+ | * The true origin of ANG is 34 degrees south | ||
+ | * This is consistant across all 8 zones in Australia | ||
+ | |||
+ | == +lon_0 == | ||
+ | * Description: | ||
+ | * Value: 151e (for zone 8 only) | ||
+ | * Value description: | ||
+ | * Each zone has a different CM. ANG has 8 zones covering the width of Australia | ||
+ | * Zone 8 covers the East of NSW | ||
+ | * Zones and their CM values can be found in the table below | ||
+ | * Each zone is 5 degrees wide in longitude, starting at 113d30e and ending at 153d30e | ||
+ | |||
+ | ^Zone Number | ||
+ | |1 |113d30e | ||
+ | |2 |118d30e | ||
+ | |3 |123d30e | ||
+ | |4 |128d30e | ||
+ | |5 |133d30e | ||
+ | |6 |138d30e | ||
+ | |7 |143d30e | ||
+ | |8 |148d30e | ||
+ | |||
+ | == +k_0 == | ||
+ | * Description: | ||
+ | * Value: 1 (default value for tmerc in PROJ) | ||
+ | * Value description: | ||
+ | * ANG has a scale factor (SF) of 1 at the central meridian | ||
+ | * This means that on the CM, a measured distance on earth will equal the corresponding grid distances | ||
+ | * East and west of the CM, a measured distance on earth will be less than the corresponding grid distances | ||
+ | |||
+ | == +a == | ||
+ | * Description: | ||
+ | * Value: 6378339.78 | ||
+ | * Value description: | ||
+ | * Based on Clarke 1858 ellposid with a defined as 20,926,348 british feet | ||
+ | * A conversion factor of 0.30479947 metres per british foot = 6, | ||
+ | |||
+ | == +rf == | ||
+ | * Description: | ||
+ | * Value: 294.26 | ||
+ | * Value description: | ||
+ | * Based on Clarke 1858 ellposid where the inverse flattening was defined as 294.26 | ||
+ | |||
+ | == +to_meter == | ||
+ | * Description: | ||
+ | * Value: 0.91439841 | ||
+ | * Value description: | ||
+ | * The ANG coordinates are in yards | ||
+ | * 1 yard = 3 british feet = 0.30479947 * 3 = 0.91439841 yards per metre | ||
+ | |||
+ | == +x_0 == | ||
+ | * Description: | ||
+ | * Value: 365759.36 | ||
+ | * Value description: | ||
+ | * ANG has a false easting of 400,000 yards west of the true origin | ||
+ | * Converting into metres is 400,000 / 0.91439841 = 365,759.36 metres | ||
+ | == +y_0 == | ||
+ | * Description: | ||
+ | * Value: 731518.73 | ||
+ | * Value description: | ||
+ | * ANG has a false northing of 800,000 yards south of the true origin | ||
+ | * Converting into metres is 800,000 / 0.91439841 = 731,518.73 metres |
georef.txt · Last modified: 2021/03/11 00:31 by allchin09