geoffleyland / lua-proj

Lua binding to Proj.4

lua-proj - Binding to Proj.4

1. What?

Proj.4 is a library for converting coordinates between geographic projections.

2. How?

luarocks install proj

then

local proj = require"proj"
local nztm_to_wgs84 = proj:new("nztm", "wgs84 +degrees")
print(nztm_to_wgs84(1600000, 6000000))    -- prints "173     -36.144718099788"
print(nztm_to_wgs84:revert(173, -36))     -- prints "1600000    6016051.5467717"

local wgs84_to_nztm = nztm_to_wgs84:reverse()
print(wgs84_to_nztm(173, -36))            -- prints "1600000    6016051.5467717"

wgs84_to_nztm:set_output("nzmg")
print(wgs84_to_nztm(173, -36))            -- prints "2510588.5912768    6578340.6211262"

Extensions to Proj.4

Proj.4 projection definition strings are hard to remember, so lua-proj contains a list of shortcuts for projections I use frequently in proj/shortcuts.lua These are NZTM, NZMG and WGS84. This list could be extended (perhaps by pull request?)

When Proj.4 does projections in "degrees", like WGS84, despite most of the documentation saying "degrees", it actually means radians. Sometimes it's more convenient to work in degrees (particularly if you're reading a shapefile whose projection is in degrees). lua-proj recognises +degrees in the projection definition string and handles conversion from degrees to radians. The +degrees is removed before the string is passed on to Proj.4.

Proj.4 has the options +proj=latlong and +proj=longlat. Despite what you might think, these both expect coordinates to be passed in in the order longitude, latitude. Sometimes it's convenient (for example when parsing shapefiles) to work in latitude, longitude order. lua-proj recognises +order=longlat and +order=latlong and swaps the coordinates as necessary. As for +degrees the +order=... is removed before the string is passed on to Proj.4.

3. Requirements

• Proj.4 installed on your system
• Lua >= 5.1 or LuaJIT >= 2.0.0.
• For building, you need the proj4 headers installed. Strictly speaking, this is only for Lua, not LuaJIT, but the rock builds the binding either way.

4. Wishlist

• I don't know enough about windows to get Proj.4 installed before building and testing for AppVeyor.

5. Alternatives

• lua-proj4