# Tag Info

## Hot answers tagged datum

22

I've explored this question recently. I think people want to know what spherical radius should I use? what is the resulting error? A reasonable metric for the quality of the approximation is the maximum absolute relative error in the great-circle distance err = |s_sphere - s_ellipsoid| / s_ellipsoid with the maximum evaluated over all possible pairs of ...

22

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 to +172 since the datum extent crosses the -180/+180 International Date Line. You'll also notice the same issue for Russian datums, such as Pulkovo 1942. (Side ...

21

The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS. Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is "mean ...

18

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) and WGS (another set of realizations) use almost the same ellipsoid GRS80/WGS84, and were originally designed in the 1980s to be equivalent. Since then, NAD83 ...

17

Think of projection as seeing your location on X/Y plane. Datum defines the reference point from where all measurements were made. Say you are located somewhere and need to tell your location to someone. You would say, i am X lat and Y long. This X and Y are deterministic because they are being referred from the Datum. The other person now knows that you are ...

17

I wrote an in-depth article on this on my blog here: http://www.sharpgis.net/post/2007/05/05/Spatial-references2c-coordinate-systems2c-projections2c-datums2c-ellipsoids-e28093-confusing It covers all these concepts in a hopefully easy to understand manner, and has been peer-reviewed by several. To sum it up: A datum is a definition of the size, ...

14

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 any attribute. Their intent is typically to minimize all types of distortion thereby not eliminating it in any specific property. Jack of all trades, master of ...

12

The Map has mostly remarks in French, so I would suggest to use a French coordinate system, and not Everest (who was British). The French used coordinate systems with a prime meridian based at Paris, so EPSG:4821 Voirol 1879 (Paris) is a good start-off. With that, the 100° meridian is hitting the Malaysian coast near Kota Baru, as in your map: You can use ...

11

A projection is used to 'flatten' the ellipsoidal shape of the earth to rectangular coordinate system (i.e, to make a 'roundish' globe into a flat map). A datum is a specific, known location on or in the Earth that is used as a reference. All GIS coordinate systems use a datum as a point of reference (i.e., it's location on Earth). There are two types of &...

11

We should remember the earth is not a simple sphere, if it was, we need one datum "= One calculation system to find a point on earth", earth is more ellipsoid, but not exactly. Earth is an astronomic geoid without a regular shape, so we may have many ways to calculate coordination of a point in this irregular 3D object, with many opinions and concepts, each ...

11

The following relies on the Wikipedia article on seven-parameter Helmert transformations. The data ("double points") consist of ordered pairs ((x,y,z), (x',y',z')) where (x,y,z) are earth-centered Cartesian coordinates in the source datum and (x',y',z') are the corresponding points in the target datum, all measured in meters. The latter are presumed ...

9

First, Proj4 uses what EPSG calls the "Position Vector" version of the 7 parameter method. It's possible that GeoTrans and Leica GeoOffice use the other version which EPSG called "Coordinate Frame". Both methods are equivalent, but the rotation matrices are different and the signs of the angular parameters have to be changed. Second, thank you for ...

9

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&milestone=2.2.0 ), for which the GDAL ticket says, "This release includes some critical changes for Australian users." EPSG 9 "What's New" page says, "New ...

8

One thing to keep in mind is that lat/long is geodetic and not geocentric: If we were to calculate elevation as a radius from the center of the ellipse, our elevation lat/long would be different than our horizontal lat/long! This is why there are two different datums. The horizontal datum is just a smooth ellipse, because it's easier to do trig functions ...

8

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 cycle). Data from tidal gauges, satellites, etc. are usually processed and stored in grid files, which contain the difference in elevation between different ...

7

Just a comment on the diagram that is trying to illustrate a projection from a sphere. Rather that what is illustrated, imagine a light source at the center of the sphere. The shadow of the polygon "projected" onto a flat piece of paper outside of the sphere is in essence a type of projection. To me the diagram is implying a projection is like a reflected ...

7

The heights on google earth refer to EGM96 and are, therefore, Geoidal heights. The lat/long are referred to the WGS 84 ellipsoid.

7

To begin with I would recommend you read this tutorial on georeferencing in QGIS http://docs.qgis.org/2.2/en/docs/training_manual/forestry/map_georeferencing.html Yes you can georeference this map. I would recommend you use a good quality flat scan of the map if possible. Avoid a crease running through the image. Accurate georeferencing depends on the ...

7

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 converge for nearly antipodal points. My library GeographicLib is accurate to about 15 nanometers and converges everywhere; the algorithm is published in my paper ...

6

EPSG has added it as 6647. At Esri, I have it in a development build, but it's not in ArcGIS 10.3 (hopefully 10.3.1). Here's the Esri WKT for it: VERTCS["CGVD2013_height",VDATUM["Canadian_Geodetic_Vertical_Datum_of_2013"],PARAMETER["Vertical_Shift",0.0],PARAMETER["Direction",1.0],UNIT["Meter",1.0],AUTHORITY["EPSG",6647]] This document on height ...

6

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, geography, etc. The US military started the process by using the same name "World Geodetic System 84" for an earth gravitational model, an ellipsoid, and a ...

6

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 (< 0.3%). If you desire an oblate spheroid then yes use VF (Vincenty's Formula). For flat I am not sure it matters, do you mean a plane? If so then just use ...

6

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 mention of any vertical data. The only time you might see altitude or elevation mentioned is if the projection allows you to set its perspective point like ...

6

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, but to do a precise transformation, you need to use a NTv2 transformation grid. While you can currently manually add the grid transformation file (GSB files), I ...

6

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-navigation/datum-modernisation for a summary of the codes. However, the information about how to transform between GDA94 and GDA2020 – by either a 7 parameter ...

5

Your input values are in the projected coordinate reference system RD New, but you're using the well-known ID for the Amersfoort geographic coordinate reference system. Change the input spatial reference ID to 28992. Also switch the "Transform Forward" radio button to "True". The transformation is from Amersfoort to WGS 84, so you want to use it in its "...

5

Disclaimer: I work at the British Geological Survey I made an internal enquiry on this issue, to see if we had any further information on the Kenya maps, and this answer is a summary of the information received. First off, it's important to note that these maps were hand drawn in the Kenya office and would have used the Kenya topography maps as their base. ...

5

Regarding the javascript to SQL, this is probably how you would handle that: SELECT FromX, FromY, CASE WHEN FromX > 180 THEN NULL ELSE FromX * 0.017453292519943295 * 6378137.0 END AS mercatorX_lon2, CASE WHEN FromY > 90 THEN NULL ELSE 3189068.5 * LOG((1.0 + SIN(FromY * 0.017453292519943295)) / (1.0 - SIN(FromY * 0....

5

There are a very large number of map projections to choose from. Which ones are "proper" really depends on what you mean by "proper". Every single one of them introduces some kind of geometric distortion (such as distance, direction or area), yet each also usually preserves some kind of geometric quality. What quality/quantity do you wish to keep "proper"? ...

5

WGS 1984 has had several "releases". I'm not sure whether or not to call them re-adjustments. WGS 1984 is loosely tied to the International Terrestrial Reference Frame (ITRF), maintained by IERS. The first transformation, WGS_1984_(ITRF00)_To_NAD_1983, assumes that WGS 1984 is the one tied to ITRF00 and the NAD 1983 realization is CORS96 or similar. That ...

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