Richard Duivenvoorde writes:
> I think we should never copy the characters, only numbers ( well,
> except when using W/S/E/N :-) )..
Having thought a bit more this seems very tricky.
> I also understood from somebody that when using latlon coordinates the
> minus sign is actually redundant, as you should use the WSEN?
I don't think it's that simple, and looking at EPSG I'm sure it's not!
(qgis should not be tied to EPSG; it should follow any defined CRS,
custom or proj-provided but I understand most are from EPSG.)
Sometimes people write any of the following
(42, -71)
(-71, 42)
42N 71W
for geographic. And sometimes DM, DMS. If we are going to try to align
with EPSG:4326, I think that means the first form. But looking at
https://epsg.org/crs_4326/WGS-84.html
it says that the parameter is in degrees [supplier to define
representation], and that latitude it positive north, longitude positive
east.
Degrees are given as
https://epsg.org/unit_9122/degree-supplier-to-define-representation.html
which means that formally, we must define a representation among the
example choices of decimal, DMSH. Other degree formats are just numbers
and numbers with hemispheres:
https://epsg.org/unit_9102/degree.html
https://epsg.org/unit_9116/degree-hemisphere.html
So I guess that means qgis could choose any of
D DH DM DMH DMS DMSH
where H means no negative sign and N/S E/W, lack of H means numbers that
could be negative and no N/S/E/W.
It is pretty obvious which representation is chosen by how it's
printed. It's clear that for cut/paste for computer use, 42,-71 is
best, and for human use it depends.
Looking beyond WGS84, I picked ITRF2014 and that is also EPSG:9122
(which I find surprising):
https://epsg.org/crs_7912/ITRF2014.html
>> Part of the underlying trouble is that in qgis/proj we blur:
>> datum
>> projection
>> into
>> CRS
>> except that some CRS are datum only, and some are datum+projection.
>
> But isn't that the 'difference' in what QGIS (in the CRS dialog) calls
> 'Geographic Coordinate Systems' vs 'Projected Coordinate Systems'?
Yes, but in opening a project and clicking around this is really not
obvious. I know which are which, but it's really subtle.
Someone commented about in US usage Northing preceding Easting, and I
think meant for UTM, but might have also meant for State Plane
Coordinates. I just checked a survey plan (MA SPC Mainland, which is
LCC) and sure enough N is given before E. That doesn't mean it's right
in all cases; GIS, geodesy, surveying are all distinct subcultures.
Looking up Mass SPC, I find:
https://epsg.org/crs_6491/NAD83-2011-Massachusetts-Mainland.html
and that is clear:
Cartesian 2D CS. Axes: easting, northing (X,Y). Orientations: east,
north. UoM: m.
which means easting has to come first, and the use of E and N is
non-compliant with the EPSG definition. Use of E/N to label those also
is fundamentally different than N/S in latitude: it's a label for whcih
coordinate is which, not a replacement for a sign bit.
My impression is that while people often write xxx.xxxN, xxx.xxxE, it is
always easting, northing when used as a coordinate in a machine-readable
way.
> So: that would make (?): if crs.isGeographic() [0] we should/could use
> WSEN, and if not then we should NOT use WSEN (and optionally x/y if
> that is more clear)... except when copying coordinates, because that
> would mess up parsers :-)
It comes back to the purpose of this. I would say:
- By default the rules in the CRS should be followed. That means N/S
can be used only if the underlying degree reference is EPSG:9122.
But then it's optional.
- Copy coordinate is intended to produce something machine-readable.
That means just numbers, to the extent that is reasonable.
- The display format should match what is copied, to follow the
Principle of Least Astonishment.
- If someone wants a richer display of coordinates in many formats,
that should perhaps be another right-click menu item.
My vote is for just numbers, like we have now, so the only thing
changing is making the axes compliant with the CRS definition.
And, perhaps either a preference to select a degree represenation (6
choices!), or multiple entries in the menu. But default should be plain
decimal degrees, IMHO.
I would be ok with the chosen degree representation being used even if
the CRS specifies 9102 instead of 9122.
I would also be ok with a preference to add E/N letters to coordinates
that are defined to mean Easting and Northing by the CRS. Default off.
Hope this helps
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