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
11
votes
Vertical Datum conversion of a raster with gdal
Convert from (EGM96) geoid vertical datum to (WGS84) ellipsoid vertical datum:
gdalwarp -s_srs "+proj=longlat +datum=WGS84 +no_defs +geoidgrids=egm96_15.gtx" -t_srs "+proj=longlat +...
- 111
10
votes
Accepted
Calculating elipsoidal length of line in PyQGIS
If you use just QgsDistanceArea.measureLength(feat), you get an error: first argument of unbound method must have type 'QgsDistanceArea'. That means you need to create an object of QgsDistanceArea
d = ...
- 76.1k
8
votes
Accepted
QGIS 3.6.1 problems with CRS
Following on Ian Turton's comment... Prior to performing ANY geometry calculations or analysis on a layer(s), the layer(s) MUST be 1) projected to the desired CRS, and 2) that CRS must be the same for ...
- 8,104
8
votes
Calculating elipsoidal length of line in PyQGIS
This is my answer:
layer = iface.activeLayer()
features = layer.getFeatures()
d = QgsDistanceArea()
d.setSourceCrs(layer.crs(), QgsProject.instance().transformContext())
d.setEllipsoid(QgsProject....
- 425
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
6
votes
Calculating elipsoidal length of line in PyQGIS
One way could be:
d = QgsDistanceArea()
d.setEllipsoid('WGS84')
layer = iface.activeLayer()
features = layer.getFeatures()
for feat in features:
print(
d.convertAreaMeasurement(
...
- 1,041
5
votes
Accepted
Do latitude/longitude coordinates ever refer to "real" earth, not idealized sphere or ellipsoid?
With these sorts of questions, the issue isn't so much with the earth or the reference system, but with the definition of "real." Angular coordinates only make sense in an axiomatic framework, with ...
- 3,101
5
votes
Why inverse flattening is zero in esri prj file for spheres?
Esri first proposed the WKT format for OGC around (estimating) 1998 using what we'd already developed in-house. The SPHEROID format has to support both ellipsoids and spheres. It is easier to allow ...
- 18.8k
4
votes
Vertical Datum conversion of a raster with gdal
This answer isn't a single command, but I'll put it in to get the ball rolling. Use gdalwarp to resample the geoid grid, then gdal_calc.py to shift the original raster.
gdalwarp -s_srs epsg:4326 -...
- 3,101
4
votes
Converting LiDAR ellipsoidal heights to orthometric heights?
PDAL can do this with its filters.reprojection capability, which is based on the vertical datum transformation capabilities of Proj.4.
pdal translate input.las output.las reprojection
--filters....
- 3,983
4
votes
Accepted
Correct area calculations: When to use planimetric, when ellipsoidal?
Ellipsoidal measurements are to be used for larger areas and lines. In otherwords, used at lower map scale representation. Example: A county boundary, district, state boundaries, a flight path or ship ...
- 173
4
votes
Accepted
Calculate ellipsoidal area for a projected layer in PyQGIS 3
If I´m not mistaken, you will need to provide a transformation context and set the layers source CRS to the QgsDistanceArea() object accordingly. Try:
...
def calculate_area(in_lyr_name, ellipsoid, ...
- 29.3k
4
votes
Accepted
CRS for Ellipsoid Measurements
More information on your CRS can be found here: https://epsg.io/26917
There you can also see that EPSG:26917 uses Ellipsoid: GRS 1980 by default, and is a projected CRS, with UoM in meters.
GIS Stack ...
- 1,156
4
votes
Getting accurate distance measurements using EPSG:3857 in QGIS
Problem of projections and accuracy of measurements:
Since EPSG:3857 (and every Mercator projection) heavily distorts lengths (the more so the closer you get to the poles), measurements in this CRS ...
- 67.2k
3
votes
Accepted
Is distance between lines of latitude same on ellipsoid?
The mathematical solution belongs rather to https://math.stackexchange.com/ but a simple image is challenging your intuition. Look at the ellipsoid and lines at angles of 0, 45, and 90 degrees. What ...
- 63.2k
3
votes
QGIS Identify feature Cartesian area in degrees
The Cartesian area is dependant on the layer's CRS. It's a "raw" value, testing the coordinates as 2d numbers only. So for geographic coordinates systems with coordinates as degree values, it's ...
- 27.4k
3
votes
Accepted
Is kml altitude mode "absolute" equivalent to altitude over WGS84 ellipsoid?
I recommend you to read this article.
An extract:
Height can be measured in two ways. The GPS uses height (h) above the
reference ellipsoid that approximates the earth's surface. The
...
- 13.2k
3
votes
Accepted
Adding EPSG:7019 (GRS 1980) to PostGIS
GRS 1980 is defined by two measurements:
Semi-major axis = 6378137
Inverse flattening = 298.257222101
With this, you've defined an ellipsoid, but you do not have a coordinate system. Where is Europe ...
- 5,378
2
votes
Which of EGM96 geoid or WGS84 ellipsoid fits the earth better?
The answer depends on what you are interested in and therefore what you mean by 'earth's surface'. The Geoid is the equipotential surface (in terms of gravitational potential). The ellipsoid is a ...
- 109
2
votes
Meaning of "Elevation above surface of ellipsoid"?
HAE, or Height Above Ellipsoid, or "elevation above surface of ellipsoid", is commonly the form of altitude a GPS receiver returns. It differs from AGL (height above ground level) and MSL (...
- 121
2
votes
Making ellipse geometry from some known parameters using PostGIS?
I would go with CIRCULARSTRING. You already know the coordinates of start point and end point, and the coordinates for the peak radius can be easily calculated.
Thus, the half of your desired ...
- 4,212
2
votes
Do different ellipsoids have different location of centre?
If two ellipsoids share the same origin and axes (both use Greenwich and the same North pole), but have different axes lengths, only the geodetic latitude will change between the two. This is true ...
- 18.8k
2
votes
Accepted
How can I use GK + ETRS89 projected data in QGIS or Proj?
You have to create your own custom CRS using the correct ellipsoid.
DHDN projections use +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7.
ETRS89 projections use +ellps=GRS80 +towgs84=...
- 10.8k
2
votes
Creating points along line at ellipsoidal distance
I would believe that you need to project the line to a azimuthal equidistant system, centered on the line origin.
Proj4: +proj=aeqd +lat_0=LINE_ORIGIN_LAT_HERE +lon_0=LINE_ORIGIN_LON_HERE +x_0=0 +y_0=...
- 13.9k
2
votes
Accepted
Bessel ellipsoid definition for LV95 (EPSG:2056)
I found the authoritative source with the help of @swiss_geoportal and @swisstopo on Twitter.
It's an ordinance called Verordnung des Bundesamtes für Landestopografie über Geoinformation (GeoIV-...
- 133
2
votes
How to set global setting ellipsoid in QGIS?
The setting to use no ellipsoid for measurements isn't directly available as a global default setting for all projects. However, you can achieve the same effect by creating a template project with the ...
- 24.5k
2
votes
Accepted
QGIS Buffer distance vs Distance in Distance Matrix
Buffer tool uses the CRS of the layer.
We can't have an "ellipsoidal distances" projection.
So, if you want to draw buffers with the ellipsoidal distance in QGIS, the most similar that you can do is ...
- 13.9k
2
votes
Accepted
PyQGIS - get ellipsoid acronym of coordinate system of layer
Yes, you can get the ellipsoid acronym from a CRS object (QgsCoordinateReferenceSystem).
For instance:
QgsCoordinateReferenceSystem("EPSG:25832").ellipsoidAcronym()
or, from a layer:
layer....
- 36k
Only top scored, non community-wiki answers of a minimum length are eligible