[talk-au] New CC-BY datasets due Monday 28 September on Government 2.0 Taskforce website

Alex (Maxious) Sadleir maxious at gmail.com
Wed Sep 30 13:44:02 BST 2009


On Wed, Sep 30, 2009 at 10:02 PM, Dan O'Hara <detectist at yahoo.com.au> wrote:
> Here is some info on the ACT grid
>
> "The ACT grid is a Transverse Mercator map projection that uses the
> longitude of Mt Stromlo Trig Station as its central meridian.  It is based
> on the Australian Geodetic Datum 66 (AGD66), which is modified to take
> advantage of the ACT’s limited east-west dimension and account for scale
> differences caused by the ACT’s height above sea level.  The resulting ACT
> grid effectively can be treated as a plane (rather than geodetic) system of
> coordinates using the formulae of plane trigonometry, without the need to
> apply scale factors, grid convergence, arc-to-chord, or sea level
> corrections. As a result, for all but the most accurate work in the ACT,
> terrestrial, grid and plane measurements can be taken as being identical."
>
> http://www.actpla.act.gov.au/tools_resources/maps_land_survey/surveying_data/surveyors_information/coordinate_system
>
> I think the reason you didn't get a match is because the grid reference is
> in yards not metres.
Aha! Thank you!
Using the downloadable tool they provide, I get the correct results
and they match what I get when I run proj4 (via the definition at
http://spatialreference.org/ref/sr-org/6628/ ):
>>cs2cs +proj=tmerc +lat_0=0 +lon_0=149.00929483 +k=1.000086 +x_0=200000 +y_0=4510193.494 +ellps=aust_SA +units=m +no_defs +towgs84 +to +proj=latlong +datum=WGS84

211550.06 600190.55

149d8'10.668"E  35d18'57.41"S 22.802

= -35.31595, 149.1363 and my guess was -35.31415,149.1374

I did stare at it a while, hoping that adding the position of Mount
Stromlo or something would give me the result in Excel. It does say
"units=m" so I don't think the yards come into it (still doesn't
explain the huge difference between ACT and AMG). But I suppose if
someone wants to import this data, they could use PostGIS.




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