Differences

This shows you the differences between two versions of the page.

--- georef [2021/03/10 17:13] – [Coordinate Conversion] allchin09
+++ georef [2021/03/11 00:31] (current) – [Coordinate Conversion] allchin09
@@ Line 107: / Line 107: @@
 Handy tables of parameters can be found [[https://en.wikibooks.org/wiki/PROJ.4|here]]
+=== Coordinate coversion for 1 inch to the mile millitary maps (ANG) ===
 'Clarke Coordinates' or the 'ANG' (Australian National Grid) have the following parameters:
@@ Line 128: / 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 151: / Line 164: @@
 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
-^Parameter    ^Parameter Description          ^Parameter Value  ^Value Description    ^Value Comment              ^
+== +y_0 ==
-|+proj        |Projection name                |tmerc            |Transverse Mecator   |Type of project used in ANG|
+  * Description: False northing
-|+lat_0       |Latitude of projection centre  |34s              |||
+  * Value: 731518.73
-|+lon_0       |Longitude of projection centre |151e             |||
+  * Value description:
-|+k_0         |Scale factor                   |1                ||1 is also default value|
+    * ANG has a false northing of 800,000 yards south of the true origin
-|+a           |Semimajor radius of the ellipsoid axis |6378339.78||Value in british feet converted to metres|
+    * Converting into metres is 800,000 / 0.91439841 = 731,518.73 metres
-|+rf          ||294.26|||
-|+to_meter    ||0.91439841|||
-|+x_0         |False easting                  |365759.36|||
-|+y_0         |False northing                 |731518.73|||