# Tag Info

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### 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) ...

### 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 +...
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### 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 = ...
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### 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 ...

### 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....

### 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 ...

### 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.

### 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( ...
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### 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 ...

### 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 ...

### 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 -...

### 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....
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### 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 ...
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### 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, ...
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### 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 ...

### 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 ...
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### 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 ...

### 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 ...
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### 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 ...
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### 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 ...

### 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 ...

### 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 (...

### 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 ...

### 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 ...
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### 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=...

### 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=...
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### 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-...

### 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 ...