Wed 19 Jun, 2024 5:53 pm
Wed 19 Jun, 2024 6:07 pm
Thu 20 Jun, 2024 6:45 pm
Fri 21 Jun, 2024 8:32 am
icefest wrote:I often draw my own magnetic north lines on my maps!
Fri 21 Jun, 2024 6:07 pm
Not a recognised geodetic system. Uses spherical development of ellipsoidal coordinates. Relative to WGS 84 / World Mercator (CRS code 3395) gives errors of 0.7 percent in scale and differences in northing of up to 43km in the map (21km on the ground).
Off-track wrote:Tom probably knows more about this than I do. His program https://maps.ozultimate.com/archive/08_print/# for NSW has a scale bar that changes size with the grid spacing; but the printed area does not change, so users will get a slightly different map scale depending on latitude.
Off-track wrote:Maybe it is possible to code in an arrow or statement of magnetic declination in each map printed from programs like Tom’s?
Fri 21 Jun, 2024 9:34 pm
ribuck wrote:If you're using your phone for the map, why not use the phone's compass? It automatically corrects for grid/magnetic, wherever you are in the world.
Sat 22 Jun, 2024 8:35 am
Sat 22 Jun, 2024 11:36 am
Sat 22 Jun, 2024 12:01 pm
rcaffin wrote:the paper map and compass is for contingency, when electronics / batteries fail.
May I suggest a slightly different PoV?
The paper map and compass is for real use because electronics / batteries fail.
Sun 23 Jun, 2024 5:13 pm
It's not hard to look up the declination for your area if you need it - the free maps from SS have it listed.
Mon 24 Jun, 2024 10:40 am
Mon 24 Jun, 2024 4:23 pm
Off-track wrote:Actually they don't.
The geopdfs from spatial.nsw have only a statement like "Magnetic North is approximately 11.54°E of Grid North".
Off-track wrote:And for reasons that I do not understand, QLD and NSW gov mappers have different ideas about the relationship between (UTM) GN and TN in the same region.
Tue 25 Jun, 2024 10:37 am
Tue 25 Jun, 2024 5:12 pm
Off-track wrote:If the QTopo mapsheets are “aligned to (UTM) grid North” why does each sheet show a grid convergence angle (GN-TN angle) and why does this angle vary between mapsheets?
Off-track wrote:Are QTopo mapsheets not in a genuine UTM projection, with left and right map edges on a TN-TS alignment, and a rose as explained by USGS (https://www.usgs.gov/faqs/what-do-diffe ... c-map-mean)?
Off-track wrote:So why do some NSW Topo mapsheets vary so much in alignment from Qtopo mapsheets of the same area? Looking more into this, I think it might be because the NSW mapsheets use a different (Lambert Conformal Conical) projection; even though GDAL (hence QGIS) reports them as UTM. The NSW mapsheets themselves tell us nothing about the projection used, or the direction of TN, and the spatial.nsw websites seem (to put it politely) ambiguous on the topic (https://www.spatial.nsw.gov.au/surveyin ... rojections). But some other sites say that the NSW Topo mapsheets are indeed projected in a flavour of Lambert Conformal Conical (https://www.cbw.bwq.org.au/leaders/QTop ... pingV2.htm).
If this is correct, it would explain why the NSW Topo mapsheets are silent about True (= geographic) North. TN is a moot concept in a Southern Hemisphere Lambert Conformal Conical projection, as the meridians diverge and the North Pole is at infinity in this projection (https://pubs.usgs.gov/pp/1395/report.pdf). It might also explain why the maps are trapezoidal.
Tue 25 Jun, 2024 10:40 pm
Wed 26 Jun, 2024 2:09 pm
Wed 26 Jun, 2024 4:34 pm
Wed 26 Jun, 2024 6:13 pm
Thu 27 Jun, 2024 9:28 am
rcaffin wrote:You know, when you are deep in Wollemi NP, a degree here or there does not seem to matter very much. Rivers, creeks and spurs matter far more.
Allchin09 wrote:Now, both NSW and QLD are doing odd things with their variance numbers in this area.
Off-track wrote:As a hiker, I can not say which state’s approach is right (though tonight is “state of origin”), but it might be helpful to give the info about projection and rotation on the Topo mapsheets. Hopefully, the other errors noted by Allchin09 will also be fixed in the next release.
Thu 27 Jun, 2024 9:48 am
Wed 03 Jul, 2024 9:10 am
Wed 03 Jul, 2024 1:20 pm
I just wanted to point out that the statement of Grid-Magnetic Variation for the 2022 PDF maps downloadable from NSW Spatial Portal, under NSW Topographic Maps, appears to be wrong in the most recent edition of the maps (2022).
For example, the statement for the Lithgow map (2017 edition) is "Magnetic North is 10.4 degrees East of Grid North." This appears to be reasonably accurate (magnetic declination of 12.0°E in 2017, less grid-magnetic angle of approx 1.51°E).
The statement for the Lithgow map (2022 edition) is "Magnetic North is approximately 12.2°E of Grid North". For 2022, the value appears to be correct for the declination (ie Magnetic North is approximately 12.2°E of TRUE North), but the statement is definitely incorrect for Grid-Magnetic Variation.
The issue appear to be consistent across all of the 2022 maps, though I've only checked a sample.
For example, on the Deniliquin map, the Grid-Magnetic Variation is stated as 10.92°E for 2022 but 9.4 degrees East for 2017. The latter is correct. The former is the declination, not the Grid-Magnetic Variation .
The Grid-Magnetic variation is the important one from the point of view of adjusting compasses, so it would be good if the error could be fixed.
Thanks you for bringing this to our attention. We will investigate further and ensure that current and correct statements are published in the next version of the maps. Please note that the current catalogue of PDF maps that are available are unable to be altered. If you feel that this error will cause difficulties with navigation please make the relevant allowances or use the older version of the maps where necessary.
Thu 04 Jul, 2024 8:46 am
Thu 04 Jul, 2024 10:42 am
Off-track wrote:Thanks Tom. You have done much better than I have with nsw.spatial. (My experience is that bureaucratic minders there intervene to block all information exchange between cartographers and the map-using public). QTopo seems helpful in comparison.
I was looking at various QTopo maps and it appears that quite a few maps have incorrect Grid North/Magnetic North relative to True North.
For Zone 54 1:100k maps, all four maps I looked at (6564, 7163, 7145, 7140) had identical magnetic declination and grid convergence - 0°34´W for grid convergence and 7°38´E for magnetic declination, for a total grid magnetic angle of 9°12´E. But looking at say 6564 Massacre Inlet, the correct values should be about 4.51°E for magnetic declination and 0.79°E for grid convergence, for a for a total grid magnetic angle of 3.73°E, which is over 5° different. In any case, the grid convergences should change gradually as you go from east to west across the zone, and then sharply when you cross a zone boundary.
The 1:50k maps I checked looked like they might be correct. 7661-2 (Mc Millan Creek) and 6564-4 (Tully Inlet) were both right, as far as I can tell.
The 1:25k maps seem to have a similar issue to the 1:100k maps. The two I looked at (7661-22, 6564-44) both had 1°6´E for grid convergence and 6°4´E for magnetic declination, even though they are on opposite sides of the zone. Neither is correct - the correct figures will be very close to those for the matching 1:50k maps above.
There was a similar issue for Zone 55 - the three 1:100k maps I looked at (7740, 8840, 8240) all had identical magnetic declination and grid convergence. I didn't check the correct values, but not all can be right!
I just wanted to bring it to your attention as anyone relying on these adjustments may be considerably out, and there seems to be a possible issue across a range of maps.
The topo mapping team appreciates you bringing this to our attention and acknowledge it does look like an error on our part.
We will investigate why this occurred and attempt to rectify this when we regenerate our QTopo maps this year.
Sat 06 Jul, 2024 8:47 am
Sat 06 Jul, 2024 11:28 am
Off-track wrote:map rose useful for orientation (as well as details of grid convergence and magnetic declination)
Off-track wrote:Does anyone remember Clag Glue or Perkin's Paste
Off-track wrote:old bed sheets and pinking shears?
Sun 07 Jul, 2024 9:02 am
Mon 08 Jul, 2024 10:57 am
Tue 09 Jul, 2024 11:30 am
Wed 10 Jul, 2024 11:51 am
Off-track wrote:Firstly, from what I can see in old mapsheets, all 'old topographic or topographic survey' sheets (from Svy Corps, Natmap, Sunmap or NSW Gov) were laid out on lon-lat edges, so they were TN up. Of course some government mapping (like Forestry and National Parks Departments) used a different rotation entirely, and it varied a lot between maps, but this was usually fairly obvious from the grids (if present) and the compass roses (which on my maps were always present). But maps in the Topo series were always TN up.Then it seems that things changed with QTopo. Now the Topo geopdf mapsheets do not seem to be laid out on lon-lat edges. They do not seem to be TN up. Rather, as Tom B pointed out, they appear to be GN up. But NSW still uses TN up. If that is correct, can you say when and why the States diverged in this matter (without warning that I can see on the mapsheets)?
Off-track wrote:Secondly, I thought (wrongly it seems) that the longest line on a map compass rose would warn us of map orientation. Can we take anything from the relative length of lines in a Topo map compass rose.
MattS wrote:Happy to clarify things a little further here is you feel it would help.
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