Differences

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

--- braindump [2018/04/17 21:26] – [Geographic Names Board] allchin09
+++ braindump [2018/10/08 23:26] – [Web Mercator (EPSG:3857/EPSG:900913)] allchin09
@@ Line 128: / Line 128: @@
 ==== AGD66 ====
-The datum used on topographic maps before GDA94. 2nd Edition topographic maps use the AMG66 projection which is derived from AGD66.
+The datum used on topographic maps before GDA94. 1st and 2nd Edition topographic maps use the AMG66 projection which is derived from AGD66.
 ==== Australian National Grid (ANG) ====
@@ Line 166: / Line 166: @@
 ===== Web Mercator (EPSG:3857/EPSG:900913) =====
-Used in many popular web mapping applications (Google/Bing/OpenStreetMap). Uses spherical development of ellipsoidal coordinates. Relative to an ellipsoidal development errors of up to 800 meters in position and 0.7 percent in scale may arise. It is not a recognized geodetic system: see WGS 84 / World Mercator (CRS code 3395).
+Used in many popular web mapping applications (Google/Bing/OpenStreetMap). Uses spherical development of ellipsoidal coordinates. Relative to an ellipsoidal development errors of up to 40 km may arise. It is not a recognized geodetic system: see WGS 84 / World Mercator (CRS code 3395).
+For details on the issues using Web Mercator for geospatial applications, see the report from the US NGA (National Geospatial-Intelligence Agency) on [[http://earth-info.nga.mil/GandG/wgs84/web_mercator/(U)%20NGA_SIG_0011_1.0.0_WEBMERC.pdf|Implementation Practice
+Web Mercator Map Projection]]
 ===== Universal Transverse Mecator (UTM) =====
@@ Line 183: / Line 186: @@
 ^Parameter              ^ Clarke 1858   ^ ANS      ^ GRS80       ^ WGS84       |
-^Major Semi Axis        | 6378293.645   | 6378160  | 6378137     | 6378137     |
+^Major Semi Axis (m)    | 6378293.645   | 6378160  | 6378137     | 6378137     |
 ^Inverse Flattening     | 294.26        | 298.25   | 298.2572221 | 298.2572236 |
 ^Used in                | ANG           | AGD, AMG | GDA, MGA    | GPS         |