Differences

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--- georef [2021/03/10 22:37] – [Coordinate Conversion] allchin09
+++ georef [2021/03/11 00:31] (current) – [Coordinate Conversion] allchin09
@@ Line 131: / Line 131: @@
 | scale factor   | 1             |              |
-Other relevant values
-| zone 8      |
+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:
@@ Line 190: / Line 200: @@
 |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
-^Parameter    ^Parameter Description          ^Parameter Value  ^Value Description    ^Value Comment              ^
+== +a ==
-|+proj        |Projection name                |tmerc            |Transverse Mecator   |Type of project used in ANG|
+  * Description: Semimajor radius of the ellipsoid axis
-|+lat_0       |Latitude of projection centre  |34s              |||
+  * Value: 6378339.78
-|+lon_0       |Longitude of projection centre |151e             |||
+  * Value description:
-|+k_0         |Scale factor                   |1                ||1 is also default value|
+    * Based on Clarke 1858 ellposid with a defined as 20,926,348 british feet
-|+a           |Semimajor radius of the ellipsoid axis |6378339.78||Value in british feet converted to metres|
+    * A conversion factor of 0.30479947 metres per british foot = 6,378,339.78m
-|+rf          ||294.26|||
-|+to_meter    |Horizontal unit value          |0.91439841|Length of a yard in metres |Yard used on coordinate grid|
+== +rf ==
-|+x_0         |False easting                  |365759.36|||
+  * Description: Inverse flattening of the ellipsoid
-|+y_0         |False northing                 |731518.73|||
+  * 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