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georef [2018/10/10 16:07]
allchin09 [Coordinate Transformation]
georef [2021/03/10 13:31] (current)
allchin09 [Coordinate Conversion]
Line 74: Line 74:
 Killet Software has an [[http://www.killetsoft.de/p_ntta_e.htm|NTvtTools]] package for developing and processing NTv2 files in both binary and ASCII formats. A trial version is freely available and a full licence currently costs 160 EUR. Killet Software has an [[http://www.killetsoft.de/p_ntta_e.htm|NTvtTools]] package for developing and processing NTv2 files in both binary and ASCII formats. A trial version is freely available and a full licence currently costs 160 EUR.
  
 +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 ==== ==== Coordinate Conversion ====
  
-[[https://proj4.org/|PROJ]] is a useful program here.+[[https://proj.org/|PROJ]] is a useful program here
 + 
 +Note that PROJ is being updated and dramatically changed - a useful overview [[https://www.gaia-gis.it/fossil/libspatialite/wiki?name=PROJ.6|here]].
  
 Handy tables of parameters can be found [[https://en.wikibooks.org/wiki/PROJ.4|here]] Handy tables of parameters can be found [[https://en.wikibooks.org/wiki/PROJ.4|here]]
  
-'Clarke Coordinate' or the 'ANG' (Australian National Grid) have the following parameters:+=== Coordinate coversion for 1 inch to the mile millitary maps (ANG) === 
 + 
 + 
 +'Clarke Coordinates' or the 'ANG' (Australian National Grid) have the following parameters:
  
 ^Parameter       ^ Value         ^ Units        ^ ^Parameter       ^ Value         ^ Units        ^
Line 100: Line 130:
 | zone latitude origin | 34S     | degrees      | | zone latitude origin | 34S     | degrees      |
 | scale factor   | 1                          | | scale factor   | 1                          |
 +
 +
 +On these maps, Sydney Observatory was used as a datum for triangulation, with the following values:
 +^Format ^Latitude          ^Longitude         ^
 +|DMS    |S 33d 51m 41.10s  |E 151d 12m 17.85s |
 +|DD     |S 33.861417d      |E 151.204958d     |
 +
 +The [[http://www.ga.gov.au/ngrs/imf.jsp?site=ngrs|NGDB]] provides the following values of the observatory in different datums:
 +^Datum  ^Latitude            ^Longitude           ^Easting       ^Northing   ^
 +|Clarke |S33°51' 41.107"     |E151°12' 17.993"    |420746.19     |816789.65  |
 +|AGD66  |S33°51' 40.2924"    |E151°12' 12.3136"   |333809.38     |6251769.51 |
 +|AGD84  |S33°51' 40.3241"    |E151°12' 12.2557"   |333807.91     |6251768.51 |
 +|GDA94  |S33°51' 34.610232"  |E151°12' 16.470597" |333913.76     |6251959.42 |
  
 Note: Note:
Line 120: Line 163:
 Which returns: E=450,666.78 N=830,202.64 Which returns: E=450,666.78 N=830,202.64
  
 +An explaination of parameters is as follows:
 +=== Parameters ===
 +
 +== +proj ==
 +  * Description: Projection name
 +  * Value: tmerc
 +  * Value description:
 +    * [[https://proj.org/operations/projections/tmerc.html|Transverse mecator]]
 +    * Projection used by ANG from June 1936
 +    * Note: prior to 1936, a polyconic projection was used
 +
 +== +lat_0 ==
 +  * Description: Latitude of projection centre or true origin (TO) of projection.
 +  * 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: Longitude of projection centre or true origin (TO) of projection. Also called central meridian (CM).
 +  * 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  ^Western Longitude   ^Eastern Longitude ^Zone Central Meridian^
 +|1            |113d30e             |118d30e           |116de                |
 +|2            |118d30e             |123d30e           |121de                |
 +|3            |123d30e             |128d30e           |126de                |
 +|4            |128d30e             |133d30e           |131de                |
 +|5            |133d30e             |138d30e           |136de                |
 +|6            |138d30e             |143d30e           |141de                |
 +|7            |143d30e             |148d30e           |146de                |
 +|8            |148d30e             |153d30e           |151de                |
 +
 +== +k_0 ==
 +  * Description: Scale factor at the central meridian
 +  * 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: Semimajor radius of the ellipsoid axis
 +  * 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 = 6,378,339.78m
 +
 +== +rf ==
 +  * Description: Inverse flattening of the ellipsoid
 +  * Value: 294.26
 +  * Value description:
 +    * Based on Clarke 1858 ellposid where the inverse flattening was defined as 294.26
 +
 +== +to_meter ==
 +  * Description: Horizontal unit value for output coordinates
 +  * 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: False easting
 +  * 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: False northing
 +  * 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.1539187644.txt.gz · Last modified: 2018/10/10 16:07 by allchin09