22
votes
Accepted
why does the North American Datum not actually include north america in it's boundary?
This is a rendering error in the spatialreference.org website and a common issue for GIS software.
The stated longitude extent covers from -52 to +172. This should actually be -52 to -180 and +180 ...
- 5,378
19
votes
Accepted
Difference between datum and ellipsoid for geodetic coordinates?
No, a datum and ellipsoid are not equivalent. For a loose definition, think of the ellipsoid as defining size and shape. The datum then fixes that ellipsoid to the earth.
NAD83 (various realizations) ...
- 18.8k
14
votes
Accepted
What is my coordinate system preserving?
From the links posted as comments I identified a couple of misconceptions I had about projections and gathered this quick summary.
It should be mentioned that many projections don’t truly preserve ...
- 696
9
votes
Transition to GDA2020 in QGIS?
QGIS depends on GDAL for a lot of its spatial data management. GDAL 2.2.0 should include GDA2020, as it includes updates for EPSG 8.9 and EPSG 9 (see https://trac.osgeo.org/gdal/query?group=status&...
- 7,922
8
votes
Vertical Datum and Projection
No, when doing a map projection, the vertical coordinates are unaffected. If you look at the math behind a map projection (cf. Snyder's Map Projections: A Working Manual). you'll see that there's no ...
- 18.8k
8
votes
What is MHHW Mean Highest High Water, in terms of datums/coordinate systems?
Mean Highest High Water is a reference surface corresponding to the height of high tides, averaged over a certain period of time (usually 19 years, a value close to a Metonic Cycle and a Lunar Node ...
- 3,806
7
votes
What datum (reference ellipsoid) does Google Earth use?
The heights on google earth refer to EGM96 and are, therefore, Geoidal heights. The lat/long are referred to the WGS 84 ellipsoid.
- 71
7
votes
Which is the most accurate method of measuring distance?
Let me focus just on the question of the most accurate way to determine
distances on the WGS84 ellipsoid.
The accuracy of Vincenty's method is about 0.1 mm providing it converges.
It fails to ...
- 3,241
7
votes
Transition to GDA2020 in QGIS?
To add to the answer of Son of a Beach, the definitions of GDA2020 CRSs have been added to EPSG, and these have been brought into Proj4 and GDAL. These CRS definitions are good enough for many uses, ...
- 13.4k
6
votes
Accepted
Is WGS 84 datum, coordinate reference system, or both?
Both. It's also a geographic coordinate reference system and an ellipsoid, and sometimes a spheroid, and a bunch of other related/similar terms that are used in the fields of geodesy, geomatics, ...
- 18.8k
6
votes
Accepted
Relation between Geoidal separation and antenna altitude in GGA sentence?
The geoidal separation just reports the height difference between the ellipsoidal surface and the geoid model's surface. Natively, GNSS calculates ellipsoidal height (height above ellipsoid surface) ...
- 18.8k
6
votes
Which is the most accurate method of measuring distance?
For the sphere you can use the SLC (Spherical Law of Cosines). The HF (Haversine Formula) will work for this as well. You can use both of these on the ellipsoid and errors are typically very low (< ...
6
votes
Transition to GDA2020 in QGIS?
Adding to answer by Alex Leith, the “definition” of the GDA2020 datum has been added to the EPSG registry and EPSG “Datum” codes added. See http://www.ga.gov.au/scientific-topics/positioning-...
- 61
6
votes
Understanding EPSG in WKT
For a full understanding see the specifications:
https://www.ogc.org/standards/wkt-crs
Interestingly, if you look at the EPSG registry (https://epsg.org/home.html) WKT2 definitions of those two CRS ...
- 13.3k
5
votes
Vertical datum query for coastal modeling
This will be possible with the Project tool with ArcGIS 10.4 (already available as pre-release). An additional parameter 'Vertical' allows you to specify an input and output vertical coordinate system,...
- 9,674
5
votes
Vertical datum query for coastal modeling
The offset that you have between your bathymetry and your elevation model relates to the difference between the Mean Seal Level (MSL) and the Lowest Astronomical Tide (LAT). From the well written text ...
- 6,486
5
votes
Accepted
Why does WGS84 refer to so many different things?
Suppose I decided my house was the centre of everything and I wanted to define a coordinate system based on my house. I'd call this the Baz2017 Geodetic CRS. Now I need to define the Prime Meridian ...
- 62.6k
5
votes
Accepted
Raster DEM vertical units are defined in meters, yet Arcmap displays them in feet
Sounds like somebody converted the raster values to feet, but didn't update the metadata. Arcmap doesn't actually "know" anything about the vertical units your raster is in, it's just reporting the ...
- 12.2k
5
votes
Accepted
Defining Latitude and Longitude before defining Datum?
Latitude and Longitude are defined upon an ellipsoid.
Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays ...
- 519
5
votes
What datums were used in North America before NAD27?
New England Datum of 1879
Was the first geodetic datum of this type in the United States. The
reference ellipsoid was Clarke Ellipsoid of 1866.
United States Standard Datum of 1901 (First North ...
- 31.4k
4
votes
Are Australian Height Datum and GDA94 applicable to Christmas, Lord Howe, and Norfolk islands?
The subcommittee that maintains the EPSG Geodetic Registry briefly had an Australian member who got some of the Australian data updated. Even so, these may not be the current coordinate reference ...
- 18.8k
4
votes
Accepted
Problem with MGI CRS on the Balkan
The MGI 1901 projection is ok. The Austrian surveying authority changed their MGI datum in the 1920ies, but the Yougoslav surveying authority did not follow, Hence they still surveyed in MGI 1901. The ...
- 76.5k
4
votes
Accepted
Datum shift issue between OSGB1936 and WGS84
You can take a third datasource as reference.
Set the project CRS to EPSG:3875, and load an Openstreetmap background with the Openlayers or QuickMapService plugin.
Then you can easily see which of ...
- 76.5k
4
votes
Accepted
What's the best way to handle a 'Site Datum' in QGIS?
You can do both:
At Using customized Coordinate System in ArcGIS Desktop? I have explained how to set up a local CRS based on the origin of the local system. The data in the local CRS remains ...
- 76.5k
4
votes
Accepted
Seems that there no tag for datums or coordinate system in kml file?
Quoting from here which seems to be a copy of the standard but not in PDF.
KML relies on a single Coordinate Reference System, which is not
referenced explicitly in KML files. The encoding of ...
- 80.4k
4
votes
Representing heights in GIS?
The term you are looking for is 'Vertical Datum'.
It is the same really as co-ordinates such as latitude & longitude. These X & Y measurements (be it degrees or metres) only make sense when ...
- 1,672
4
votes
Accepted
Determining Layer's DATUM and PROJECTION in "Layer Properties" (QGIS)
You can always find all corresponding information about certain projection by its standard EPSG code on spatialreference.org.
For example, your Layer 1 has EPSG code of "2203", so here is all ...
- 3,533
4
votes
Accepted
Adequate proj4string for ETRS89
Finding the proper proj4-string can be a bit funny.
Such strings provide information on both the ellipsoid and how coordinates on the surface are arranged. Deciphering your example gives us the ...
- 6,486
4
votes
Accepted
Reproject in QGIS doesn't work the way like in ArcGIS
The fact that they have exactly the same shape and are laterally offset by just a few meters suggests the conversion to a projected CRS is fine but the result has been projected on a slightly ...
- 124
4
votes
PostGIS ST_Transform seems to treat the vertical datum but not the equivalent pyproj Transformer, even so they both rely on Proj
One thing to note is the version of PROJ that is being used. PostGIS in your version is using PROJ 5.2 and the version of pyproj you are using is likely using PROJ 7.2+.
There are quite a few things ...
- 7,191
Only top scored, non community-wiki answers of a minimum length are eligible