oruxmaps. utm grid strangeness with NSW topos

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oruxmaps. utm grid strangeness with NSW topos

Postby wildwanderer » Sun 14 Jun, 2020 9:48 pm

Hoping one of the resident map gurus can explain this.

In orux maps the grid reference are out about 20m from the grid marked on the nsw topo. This is most apparent at the map edges. As one goes closer to the map centre the error gets smaller.

The following screenshot shows the error. Grid reference is at bottom. Bullseye cursor is the grid reference position.

Screenshot_20200614-213239.png
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby michael_p » Sun 14 Jun, 2020 10:44 pm

By my reckoning you are just off the Ingar Trail on the Jamison 8930-2N map. The Jamison sheet is MGA (1966) not GDA (WGS84). If you look down in the bottom left corner of the downloadable pdf map MGA is printed on the map border (refer picture below).

My experience with the downloadable geopdf files is that the pdf is georeferenced as GDA. It's printed on the bottom right of the map border and is in the metadata (see output of metadata below). Some older maps that have not been updated to GDA are still MGA. I think Oruxmaps takes the map datum from the pdf so it would be referencing the image as GDA.

Try changing the map datum to MGA and see how you go.

Michael.
jamison mga.jpg
jamison mga.jpg (187.68 KiB) Viewed 7336 times


Metadata from the PDF file:
Driver: PDF/Geospatial PDF
Files: 8930-2N JAMISON.pdf
Size is 6767, 3484
Coordinate System is:
PROJCS["GDA_1994_MGA_Zone_56",
GEOGCS["GCS_GDA_1994",
DATUM["Geocentric_Datum_of_Australia_1994",
SPHEROID["GRS_1980",6378137.0,298.257222101]],
PRIMEM["Greenwich",0.0],
UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],
PARAMETER["False_Easting",500000.0],
PARAMETER["False_Northing",10000000.0],
PARAMETER["Central_Meridian",153.0],
PARAMETER["Scale_Factor",0.9996],
PARAMETER["Latitude_Of_Origin",0.0],
UNIT["Meter",1.0]]
GeoTransform =
243936.077432269, 4.232208796921401, 0.1079758749961432
6262520.787086848, 0.1076744821363498, -4.232683836414725
Metadata:
CREATION_DATE=D:20171020132244+10
CREATOR=Esri ArcMap 10.3.0.4322
NEATLINE=POLYGON ((244087.729816912178649 6248020.518988867290318,244088.120437954727095 6263041.616802841424942,269957.185805182205513 6263040.647875521332026,269957.807962849154137 6248020.708077747374773,244087.729816912178649 6248020.518988867290318))
Corner Coordinates:
Upper Left ( 243936.077, 6262520.787) (150d14' 8.81"E, 33d44'47.31"S)
Lower Left ( 244312.265, 6247774.117) (150d14' 8.03"E, 33d52'45.89"S)
Upper Right ( 272575.434, 6263249.420) (150d32'41.52"E, 33d44'47.20"S)
Lower Right ( 272951.622, 6248502.750) (150d32'42.46"E, 33d52'45.86"S)
Center ( 258443.850, 6255511.768) (150d23'25.19"E, 33d48'46.92"S)
Band 1 Block=6767x1 Type=Byte, ColorInterp=Red
Band 2 Block=6767x1 Type=Byte, ColorInterp=Green
Band 3 Block=6767x1 Type=Byte, ColorInterp=Blue
Last edited by michael_p on Mon 15 Jun, 2020 9:53 am, edited 1 time in total.
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby wildwanderer » Mon 15 Jun, 2020 9:48 am

Thanks Michael,

Ive had a look at the datums on orux maps. They do offer MGA 1966 but when selected that was also out 20 metres but east instead of west!

They dont offer MGA 1994. I wonder if the MGA 1994 would remove the inaccuracy?

However the difference between WGS 1984 (which is orux map default) and MGA 1994 should only be about 1.8m. But the grids displayed are 20m off in the application!
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby michael_p » Mon 15 Jun, 2020 10:16 am

OK. Seems a weird problem if it isn't the datum. It was late last night when I replied so I'll have a look at Oruxmaps on my phone and see if I can replicate the error and work out what is going wrong.

There is no MGA94 it's GDA94 which is based on WGS84 (the updated world model for GPS introduction). The difference between WGS84 and GDA94 is small and the two are, for bushwalking purposes, interchangeable.

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Re: oruxmaps. utm grid strangeness with NSW topos

Postby Mark F » Mon 15 Jun, 2020 6:16 pm

You say the distortion is smaller towards the centre. As the map is trying to convey the convex planetary surface on a plane it could be that the pdf and orux are using different/simpler mathematical formulas to account for the curvature or using a different "zero distortion point/plane" for the calculations.
"Perfection is attained not when there is nothing more to add, but when there is nothing more to remove".
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby ken333 » Tue 16 Jun, 2020 2:46 pm

I downloaded the NSW SIX ETopo map 8930-2N JAMISON.pdf file and transferred it to the oruxmaps/mapfiles folder on my Android. I started Oruxmaps and created a Waypoint at -33.75123, 150.39821, which is the Lat Lon shown on your screenshot. I loaded the ETopo map and displayed the waypoint, and centred the map on it. It shows the waypoint sitting on the 59000 line. The grid reference is shown as 56H 259000 6262385.
Edit: I have also enabled the Oruxmap grid, which closely (but not exactly) matches the grid on the pdf.
Is this different to what you got?
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Last edited by ken333 on Thu 18 Jun, 2020 4:31 pm, edited 1 time in total.
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby michael_p » Tue 16 Jun, 2020 3:22 pm

I did the same thing as Ken and got the following:
Screenshot_2020-06-16-15-08-54.png


Wildwanderer,
I have a version of the jamison etopo from 2016. I compared it to the 2017 version (which is the current downloadable version) and found that they have different boundary coords. How old is the version of the jamison map you are using? If its the older version that could explain the error.

I hope to do more testing soon. I'll post what I find.

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Re: oruxmaps. utm grid strangeness with NSW topos

Postby wildwanderer » Thu 18 Jun, 2020 4:07 pm

Hi Michael,

Looks like I have an old Jamison e topo as well. Creation date 17/5/2016

Although its strange that if your using the 2016 one as well your result is in a different location to mine. Your right of the 59 grid line... Im left.

@Ken im not gettin the same result as you either. Im left of 59 grid line.

Very weird
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby michael_p » Thu 18 Jun, 2020 7:38 pm

Wildwanderer,

I have loaded the old (2016) version of the map and the waypoint now appears some way to the west of the 59 line (see below). I suspect that the LPI became aware that there was an error with the 2016 version and released the corrected version in 2017 (even though the map has 2016 on it).
Screenshot_2020-06-18-10-03-15-jamison2016.png

As to the vagaries between different phones giving slightly differing locations for the correct map. I have no idea, maybe just a difference in rendering the image for different screen resolutions and sizes.

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Re: oruxmaps. utm grid strangeness with NSW topos

Postby wildwanderer » Thu 18 Jun, 2020 8:39 pm

Thanks Michael,

Seems indeed a map error.

I wonder how many maps are affected...

Doesn't seem like a big deal but a bit annoying in a few circumstances. Most prominent I can think of is using a GPS position to verify your on the correct ridge.

Some places (in Kanangra Boyd, for example) have multiple parallel forested ridges, look virtually identical and are in close proximity. One is the true ridge, the others are spurs but it's hard to tell from the top.

Looks like I may need to start redownloading some topos
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Re: oruxmaps. utm grid strangeness with NSW topos

Postby felixthecat » Mon 17 Aug, 2020 9:09 pm

I've recently dumped most of the maps here if that helps anyone looking at this:
https://www.dropbox.com/sh/wqv180al7ulr ... eOe8a?dl=0

I've converted some to GeoTiffs which are also in the folder.
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