Port Hacking 9129-4N Topo Map 1:25K Datum?

[WaltzingMatilda]

G'day all,

Hope you are all well! Can anyone help me to find out what Datum was used for Port Hacking 9129-4N Topo Map 1:25K 2017 Edition.

I purchased a copy printed on Tyvek. Here's the link to my map: https://www.mapworld.com.au/products/port-hacking-1-25k-topo-lpi

Thanks everyone!
Cheers,
WaltzingMatilda.

[tom_brennan]

GDA94

[WaltzingMatilda]

Appreciated! Thanks very much, Tom!
Ps: Would love to know where you found out the datum. I couldn’t find it anywhere, but I’m a newby! Keen to learn everything I can.
Thanks again and best wishes

[michael_p]

If you have a look at the bottom right corner it says GDA next to the Waratah.

Some further reading here: https://www.spatial.nsw.gov.au/surveyin ... rojections

Cheers,
Michael.

[tom_brennan]

Local knowledge (plus what michael_p said above)

The printed maps used to have detailed information on datums, projection, accuracy etc printed on the reverse side, along with the aerial photo. The GeoPDFs only have one side, so the info is missing. A bit sad.

Pretty much every large scale map in NSW from around 2004 to 2021 was in GDA94. NSW is now moving to GDA2020, which for current bushwalking purposes is similar enough to GDA94 to not matter.

Also keep in mind that for printed (flat) maps, the projection (MGA94) is as important as the datum.

[WaltzingMatilda]

Ah excellent! Thanks Michael and Tom!

[Son of a Beach]

For a commercially published map, it's rather shonky that it doesn't tell you which datum it's using, let alone which projection and coordinate system.

The fact that it says "GDA" on the bottom implies that it must be GDA94 because when that edition was made in 2017, almost nobody would have been using GDA2020, and the previous Australian equivalent datum was AGD84 and AGD66. Therefore the datum must be GDA94 (almost for certain).

However, that doesn't completely define the projection and the coordinate system. But you may not care about that. It has a grid that appears to be for a UTM coordinate system, but that grid is not parallel to the map boundaries. This implies that the grid coordinate system is not the same as the projection used for the map OR that the map has been rotated (eg, so that true north is straight up, perhaps). It's very much lacking in information about the projection and coordinate system (and datum)! Rather a poor production.

[michael_p]

^Agree 100%.

On a side note. The datum for the UTM projection can be found in the bottom left of the map window on the second line up and the second line across.

[Allchin09]

For a commercially published map, it's rather shonky that it doesn't tell you which datum it's using, let alone which projection and coordinate system.

As Tom said, the additional marginalia giving details on projection, coordiante reference system, etc was provided on the obverse of the printed maps and is missing for the e-topo versions which only include whatever was on the front.

Feedback was provided when they first introduced the e-topos, sadly it wasn't acted on and now they (NSW DCS Spatial Services, the government entity responsibly for survey and mapping) don't even supply the printed versions...

[Son of a Beach]

^Agree 100%.

On a side note. The datum for the UTM projection can be found in the bottom left of the map window on the second line up and the second line across.

Oh, yes... on the grid ticks it says "(MGA)" under the first one on each axis. I hadn't noticed that before. So MGA is UTM, GDA94 zone ??? - hmmm it still fails to include which MGA (UTM) zone the coordinates are in.

And of course those coordinates are still not in the same projection as the map itself, as the grid lines are not parallel to the edges of the map. The map projection is in which the geographic (lat/lon) grid (if it was printed) would be parallel to the edges of the map (as indicated by the same geographic (lat/lon) coordinates on each corresponding corners of the map).

As Tom said, the additional marginalia giving details on projection, coordiante reference system, etc was provided on the obverse of the printed maps and is missing for the e-topo versions which only include whatever was on the front.

Feedback was provided when they first introduced the e-topos, sadly it wasn't acted on and now they (NSW DCS Spatial Services, the government entity responsibly for survey and mapping) don't even supply the printed versions...

I guess that makes a little more sense. All the (printed) topo maps I've ever used have all this on the front, and the back is either blank, or has other useful information about the area that is not strictly map-related. Placing parts of the actual map on the back seems a little silly even before they went 100% electronic. As you say, it's quite dopey that they don't rectify this after going fully electronic. Would be trivially easy to do.

[tom_brennan]

However, that doesn't completely define the projection and the coordinate system. But you may not care about that. It has a grid that appears to be for a UTM coordinate system, but that grid is not parallel to the map boundaries. This implies that the grid coordinate system is not the same as the projection used for the map OR that the map has been rotated (eg, so that true north is straight up, perhaps). It's very much lacking in information about the projection and coordinate system (and datum)! Rather a poor production.

The map boundaries are True North. A UTM grid is only parallel to the map boundaries (True North) in the middle of a UTM zone. Elsewhere, it will diverge by the amount of the grid convergence - which used to be printed on the reverse side of the map!!!

Typically this ranges from -1.5 degrees to 1.5 degrees in NSW.

[Son of a Beach]

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