How it works
The national grid of Australia is UTM on the national datum: the Australian Map Grid (AMG) on
AGD66, then the Map Grid of Australia (MGA) on GDA94 and now GDA2020. Each zone is 6 degrees wide,
and the edge between zones 54 and 55 is the meridian 144 E, which runs through the middle of
Victoria: Ballarat is in zone 54 and Bendigo, Geelong and Melbourne are in 55. Coordinates either
side of it are on different grids, and a map, a survey or a road that crosses it has to pick one
and live with the other zone's scale error.
VicGrid puts the whole state on one grid: a Lambert conformal conic with standard parallels
36 and 38 S, where the cone cuts the ellipsoid and the scale is true, and its origin at 37 S 145 E.
Between the parallels the scale is a little small (0.99985 at 37 S), outside them a little large
(1.0005 at Wilsons Promontory, 1.0012 at the north-west corner), and it does not change along a
parallel, which suits a state wider than it is tall. Turn on Scale error to see it, and
switch to an MGA zone to compare: there the scale grows away from the central meridian, so it is
good in a strip and poor across the state.
The grid moved with the datum. Vicgrid66 (EPSG:3110) is on AGD66 and the Australian
National Spheroid, with its false origin at 2 500 000 m east, 4 500 000 m north. Vicgrid94
(EPSG:3111) keeps the same cone on GDA94 and GRS80 but takes 2 500 000 m off the false northing,
so a Vicgrid94 northing (about 2.4 million in Melbourne) can never be read as a Vicgrid66 one
(about 4.4 million), whose points are also about 200 m away on the ground because of the datum.
GDA2020 / Vicgrid (EPSG:7899) keeps the numbers of Vicgrid94; its coordinates differ by
about 1.5 m in Melbourne, how far the continent moved between the datums.
The neighbours do the same for themselves: NSW Lambert (EPSG:8058, standard parallels
30.75 and 35.75 S) and SA Lambert (EPSG:8059, 28 and 36 S). Tasmania sits inside MGA zone
55. For the whole continent, Australian Albers (EPSG:3577 on GDA94, 9473 on GDA2020) is an
equal-area conic with standard parallels 18 and 36 S; it keeps area, not shape, so its scale
along a parallel and along a meridian differ (the readout gives the parallel's).
The map is drawn in the chosen grid's own eastings and northings, so its grid lines are straight
and everyone else's bend. The readout gives the point under the pointer in every grid, through
its datum: GDA2020 to GDA94 with EPSG:8048 and GDA94 to AGD66 with PROJ's "AGD66 to GDA94 (4)"
seven-parameter transformation for New South Wales and Victoria (about 1 m; the published
distortion grids do better). src/core/vicgrid.js has the projections (Snyder 1987) and
shifts, checked against PROJ in test/check-vicgrid.mjs.
Hash keys: crs, over, shade, zones,
grat, towns, v (centre lon, lat and width in km),
pt.
- Definitions from the EPSG Geodetic Parameter Dataset as
PROJ carries it; the transformations are PROJ's.
- Snyder, J. P. (1987) Map Projections: A Working Manual, USGS Professional Paper 1395
(Lambert conformal conic, Albers equal-area conic); Karney, C. F. F. (2011) Transverse Mercator
with an accuracy of a few nanometers, J. Geodesy 85, 475.
- State borders from Natural Earth 1:10m admin-1
(public domain), made by
test/make-aus-states.py.