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georef [2018/10/12 04:09] – [Coordinate Transformation] allchin09georef [2021/03/11 00:29] – [Coordinate Conversion] allchin09
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 |s|-3.2756552|ppm| |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 ANS and GDA94 coordinates. This can then be used in QGIS to 'reproject' ANS georeferenced rasters into a GDA94 CRS.+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 ANS 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'' ''+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''
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 ==== 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        ^
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 | zone latitude origin | 34S     | degrees      | | zone latitude origin | 34S     | degrees      |
 | scale factor   | 1                          | | scale factor   | 1                          |
 +
 +
 +Sydney Observatory was used as a datum for triangulation, with the following values:
 +^Value     ^DMS                  ^DD                 ^
 +|Latitude  |33d 51m 41.10s south | 33.861417d south  |
 +|Longitude |151d 12m 17.85s east | 151.204958d east  |
 +
 +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:
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 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.txt · Last modified: 2021/03/11 00:31 by allchin09

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