# Tag Info

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Geographic coordinate systems (lat/long) are based on a spheroidal (either truly spherical or ellipsoidal) surface that approximates the surface of the earth. A datum typically defines the surface (ex radius for a sphere, major axis and minor axis or inverse flattening for an ellipsoid) and the position of the surface relative to the center of the earth. ...

83

You will obviously get better answers from textbooks, but here is an simple explanation: Map Projection: It is a method for representing a spherical or curved surface on a flat plane. Datum: It is the reference or origin based on which measurements are made.

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After struggling with this question ten years ago, and finding many confusing things written about the topic, I published a brief article in Directions Magazine that presented an answer as simply, plainly, and accurately as I could make it. The following is excerpted from that article. Reprojecting geographic features Two things must happen when you draw a ...

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In short, the distance can be in error up to roughly 22km or 0.3%, depending on the points in question. That is: The error can be expressed in several natural, useful ways, such as (i) (residual) error, equal to the difference between the two calculated distances (in kilometers), and (ii) relative error, equal to the difference divided by the "correct" (...

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wwnick's answer is correct, but it is a bit misleading in the sense that it emphasizes ellipsoid parameters and IMO understates the importance of 'the position of the surface relative to the center of the earth' - the NAD 1927 example needs to mention that the geodetic "center" of NAD27 is a base station at Meades Ranch in Kansas. One could have (and often ...

29

I discovered FlexProjector yesterday. It interactively lets one explore various world projections, tweak their parameters, and even invent new projections while displaying results on screen immediately, complete with Tissot Indicatrix (though I don't know yet if they're approximated or accurate). Flex Projector is open source (GPL2) and cross platform (...

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There can be some confusion over the difference between a Spatial Reference System (SRS) and a Coordinate Reference System (CRS), and unfortunately WGS84 is often used for both. EPSG:4326 is merely the reference number of EPSG's database entry describing a CRS. Typing 4326 into their website here gives us this: The things to note here are that 4326 refers ...

25

a. It is good practice to store a version of the data in the projection in which it was captured. Re-projection can be a lossy process, and it is important to have the original. I have a preference for storing in WGS84 for the sake of simplicity. b. Depending on your software you may need to re-project into a metre based projections such as UTM. ...

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 ...

15

Geographic projections are a way of showing the curved surface of the Earth on a flat surface like a piece of paper... From the Manifold user documentation: Earth is not an exact ellipsoid. In fact, because the Earth is such a "lumpy" ellipsoid no single smooth ellipsoid will provide a perfect reference surface for the entire Earth. The practical ...

15

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 ...

14

For detailed, beautiful and well presented information on nearly all aspects of map projections at world scales I cannot praise Carlos Furuti's Map Projection pages enough. The great amount of information there can be daunting and scares people off sometimes, so here are two noteworthy starting pages to get you hooked: Assessing and Measuring Distortion ...

13

Our strategy is to use the same for a, b and c. Reprojection is costly. (We store, analyze and display data in the projection most used by our users - our national UTM zone - or one of them actually. This being Norway, any Mercator projection is distorting much. Still, the WGS84/Google projections EPSG:4326/EPSG:900913 are the other relevant ones we have ...

13

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 donâ€™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

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 ...

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Two things are happening here. The first is the replacement of the actual latitude phi_n by the "conformal latitude" phi_nc. Think of this as distorting the ellipsoid (as specified by r_eq and e) into a perfect sphere. Because it is an ellipsoid of revolution, no change to the longitude occurs (lambda_n = lambda_nc), but the latitudes shift slightly. ...

10

Gdalwarp is the tool to reproject, you find it in Qgis under Raster->Projektionen->Transformieren or standalone in OSGEO4W. Basic command is gdalwarp -s_srs EPSG:25832 -t_srs "+proj=tmerc +lat_0=0 +lon_0=6 +x_0=2500000 +y_0=0 +k=1.000000 +ellps=bessel +units=m +nadgrids=./BETA2007.gsb +wktext" input.tif output.tif BETA2007.gsb should be in the same ...

9

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, ...

9

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 ...

8

This won't compete with wwnicks answer and not rigorous, but the visualization I present to people, when asked, is the relationship between a string connected to a ball. Changing the projection is often like moving the 'loose' end of the string around, but still connected to the same point on the ball. Changing the datum is like changing the location of the ...

8

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 ...

8

The choice of datum doesn't matter for map making (provided you change datums appropriately, of course). It does matter for transmitting coordinates and sharing data. To address your last question ("does it even make sense?"), note that UTM is really a coordinate system and as such--in addition to its grid-based zone naming system--it includes a definite ...

8

Without going into a lot of details (which can be obtained from some of the resources mentioned in the earlier answers) the standard cartographic answer is "it depends on your use and your audience." All projections distort at least one of the following: shape, area, or direction. For instance, if your analysis requires acccurate measurements of area, you ...

8

You can compare the two. In most applications I suspect the second (direct) method will be the one to choose. Accuracy of the first (iterative) method depends on the accuracy with which you do the computations and when you decide to stop iterating. It therefore can be made as accurate as the second method for all inputs where both are valid (the first ...

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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 ...

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 ...

7

As with Relet and Jonatr we use a local coordinate system if at all possible, which in our case is Albers. Our reasoning: All coordinate systems are arbitrary. By a coordinate system, we simply mean an alphanumeric system by which the positions of geographic objects can be be unambiguously described. By arbitrary, we mean there's no magic to ...

7

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 ...

7

Other replies in this thread show that some specialized datums do depend on the earth's magnetic field. However, geodetic datums are determined ultimately by the earth's gravitational field, which establishes the "geoid" (an idealized "sea level," or contour shell of gravitational equipotential). The geoid is then approximated by an ellipsoid of revolution ...

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