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What strategies, criteria, or rules do you use for selecting coordinate systems for

  • (a) storing,
  • (b) analyzing, and
  • (c) displaying GIS data?

(I humbly offer my reply to a related question about watershed analysis as an example of the considerations involved in (b).)

What are the pitfalls to watch out for?

Links to Web sites you find particularly helpful in this regard are welcome.

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I anticipate a varied set of responses because the solution might depend on what the aims of your GIS application are, so please don't hesitate to offer your answer even if others have been posted! –  whuber Oct 20 '10 at 21:10
    
@Kirk Kuykendall makes an excellent point in a Web mapping context (which could serve as a reply to this thread) in a comment at gis.stackexchange.com/q/10438/664 . –  whuber May 31 '11 at 19:21
    
The coordinate system that you use only matters if you are concerned with accuracy. Engineers and surveyors do not use UTM. –  Jerry Nov 7 '13 at 0:44
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@Jerry I strongly disagree with your point. Engineers and Surveyors are more concerned with accuracy than anybody else. This is why in most cases they chose to use a local control point as the origin for their coordinate System. But that is irrelevant, as your answer doesn't really talk about how to select a coordinate system. –  Devdatta Tengshe Nov 7 '13 at 2:51
    
Jerry, @Devdatta is correct for additional reasons: the choice of coordinate system changes the visual impression of the map, too, so more than accuracy is at stake: we need to be concerned about how map readers make sense of cartographic output and that can be strongly influenced by coordinate systems. But going back to accuracy: some (bad) choices of coordinate systems create infinite amounts of inaccuracy, implying that for just about any purpose you must be concerned about accuracy to some degree. –  whuber Nov 7 '13 at 17:29
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7 Answers 7

up vote 20 down vote accepted

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. Support for native geodetic is being added (PostGIS, SQL server). For raster analysis it can be preferable to keep the data in the format it was supplied to avoid data loss through interpolation.

c. Web mapping systems have standardised on spherical mercator, and to overlay tiles they need to be in this projection. For overlaying vector on google maps you use WGS84. If you are not targeting web mapping there will usually be a local projection that is standard.

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+1 for the advice in (a): very good point. –  whuber Jan 15 '11 at 15:40
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I am accepting this answer, despite its relative unpopularity, because apparently it alone recognizes the distinction between the format in which coordinates are stored and the formats in how they are used for analysis and processing. Although all the other answers are interesting and useful--I am grateful for them and have upvoted them all--the question does not ask about issues of distortion in projection, which seems to be the focus of many of the replies. –  whuber May 21 '11 at 20:17
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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).

alt text

Flex Projector is open source (GPL2) and cross platform (linux, mac, windows), written by Bernhard Jenny, Institute of Cartography ETH Zurich. Truly a wonderful gift to the geospatial professions and community.

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+1 Thanks for sharing this. It's so useful we shouldn't much care how accurate the Tissot indicatrices are :-), but on the sample screenshots they all look like true ellipses, which is a good sign. –  whuber Jan 12 '11 at 19:44
    
+1: A valuable resource. –  menjaraz Apr 10 '12 at 12:09
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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 to deal with. YMMV)

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Relet said it all. The same projection(s) is used for all three and rarely changed. Which one is used? The local projection, nobody here uses other stuff - in my case the local Transverse Mercator (not UTM) and very very rarely the geographic projection. –  jonatr Oct 20 '10 at 21:59
    
When you say you "use the same for ... c" and "reprojection is costly," are you arguing against use of any GIS's on-the-fly projection capabilities? –  whuber Dec 18 '10 at 21:13
    
Absolutely not. But it makes sense to store data in the projection that will be used most, if there is a clear disparity. –  relet Jan 7 '11 at 14:59
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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 using Tissot Indicatix. All the circles in the following image comprise the same area and distance on the ground. (Matt Perry has a short gdal/ogr script for generating tissot shapes here). Secondly see the Summary of projections table with thumbnails.

Mercator projection with Tissot Indicatrix

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Thanks Matt. Good references. Note, though, that Matt Perry's script only approximates the Tissot Indicatrices; in some cases it gives woefully distorted values. Both he and Carlos Furuti seem not to have noticed that one can compute a TI accurately using a tiny circle (e.g., 10 meter radius) and then just rescale it by a constant amount around its center for display on a map. But, by and large, even the distorted representation is useful. –  whuber Nov 27 '10 at 3:18
    
Thanks for the heads up and correction Bill. I had taken the assertions at face value. How does one determine if an indicatrix is valid? –  matt wilkie Nov 29 '10 at 0:28
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The shapes of the indicatrices provide a useful clue. Look at the ones where the greatest distortions occur, especially the ones that have strong eccentricities. If they are no longer ellipses--for example, if they start looking like bananas--you can be certain the calculation is invalid. The Tissot indicatrix is a second order approximation to the derivative of the projection and, as such, necessarily is always a perfect ellipse. –  whuber Dec 18 '10 at 21:11
    
Now a question of it's own: Creating an accurate Tissot Indicatrix –  matt wilkie Feb 10 '12 at 7:30
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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 need to use a projection that preserves area. Your choice will also depend on the geographic area of interest: is it large (the entire earth, an entire continent) or relatively small (a city, county, or even a region in some cases). UTM and State Plane coordinate systems (based on projections) may work well in these areas, but they are useless across an entire continent. Your audience is also important because they may have a particular projection that they are accustomed to seeing (even if they don't know it), or a particular standard that they area expecting. One thing for sure though, even if you store your dataset in geographic coordinates, I wouldn't recommend ever using GC for an actual cartographic representation unless you absolutely had to -- shape, area, and direction are ALL distored, and besides, the resulting map is likely to be ugly.

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+1 Thank you for introducing some important considerations and addressing all three parts (a, b, and c) of the question. I have been hoping to see more responses like this one. –  whuber Mar 29 '11 at 15:47
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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 how these systems were developed...they are completely contrived. Their sole purpose is to enable users to unambiguously describe an object's position in geographic space. It doesn't matter what system you use: spherical (Global Reference System) or planar (Albers, Lambert, UTM, State Plane, etc, etc...), as long as you can place objects in the correct location.

In summary:

  1. a coordinate system allows you to describe an object's geographic position,
  2. the object's position is important,
  3. how the object's position is described (i.e. coordinates) is not important.

The only real issue for us is that if data from a variety of sources are to be integrated, they must all reside in the same coordinate plane. Because of the theoretical limitations of UTM:

  1. UTM zones are limited to 3° on either side of the central meridian,
  2. mapping beyond the 3° limitation results in unacceptable aerial distortion
  3. the Yukon spans 4 UTM zones...

...and the simple fact that our users wish to be able to:

  1. integrate a variety of spatial data,
  2. create maps that span multiple UTM zones

...but don't give a hoot about the actual coordinates, we have abandoned UTM for Albers.

Instrumental in setting this standard was "Standard for the Use of Map Projections in British Columbia for Resource, Cultural and Heritage Inventories Ministry of Environment, Lands and Parks and Ministry of Forests for the Digital Data Working Group Resources Inventory Committee" (link) (how's that for an unwieldy title? :)

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That is an awesome unwieldy title. –  relet Oct 21 '10 at 13:39
    
Your assertion that "all coordinate systems are arbitrary" seems to fly in the face of most of the modern literature on projections, which shows how one can select a projection to achieve an optimal trade-off among the effects of the distortions for your intended analyses. It also seems to suggest that almost any projection would be suitable for any work, which--to put it mildly--is an unconventional stance. Have I perhaps misinterpreted what you are saying here? –  whuber Dec 18 '10 at 21:17
    
@whuber: the word "arbitrary" here is meant to convey the idea that we, the humans, are imposing a mental construct onto the reality out there. If we were using for example, a base 12 number system, the coordinate system would be completely different yet could describe the world just as well. By no means do I mean to imply it doesn't matter which system is used. In matter of fact, choice of coord-sys is often the most fundamental decision to be made. –  matt wilkie Dec 21 '10 at 19:16
    
The central point in this is that querying the same location in Albers and UTM yields different numbers, but the location in relation to others is so similar as to render the difference meaningless. Likewise area and distance calculations compared between Albers & UTM are close enough that the differences can be considered statistical noise at this scale. (We commonly deal with features in the hundreds and thousands of Km: Parks, Game Management Zones, etc.) –  matt wilkie Dec 21 '10 at 19:26
    
Thank you for the clarification. Your broad view of coordinate systems, although correct, seems to overlook the fact that many computations are possible only in certain coordinate systems or are much more efficient in certain systems. As such, even though certain systems might position points with appropriate precision, they do not describe how we want to reason about the world equally well, by any means. (E.g., base 12 makes no difference, but projected vs geographic does.) That perhaps explains why it is not paradoxical that choosing a coordinate system is an important decision. –  whuber Dec 21 '10 at 19:28
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I responded to a not :) unrelated question on qgis forum earlier...

It would depend on which CRS you are using. All coordinate systems warp reality in one way or another. they either warp the graphic depiction so that measurement is localized and more accurate for a "smallish" area, or they warp the measurement so that we like what we see. As in the US48 albers is a common "picture" of the US with a nice curve on canadian border, and Maine bigger than Texas, curve nicely centered somewhere in the midwest. However there is a relatively small area (in the center) where measurements can be made accurately. UTM and Stateplane are made for the US to localize and allow accurate measurement. Yes the geodetic measurement will always be different than the object length unless you build the object with 3d measures. I am not familiar with qgis measurement methods but you would want to use a geodetic measurment as the accurate one. It will come closest (understand that the coordinate system you are using utilizes an approximation of the ellipse of the earth where you are, and that could have quite a bit of + or - built in) to being the same measurement as the real world. Hope this helps Also don't forget that lat long is not a coordinate system, it is an angular system (thus deg units)

2010/10/21 Frederick Löbig

  • Hide quoted text - Hey list,

when I calculate the length of a polyline with the field-calculator, the value is different from the one measured with the measurement-tool. I think that this is a problem with the geodetic differenze between the real length, and the flat projected lenth.

My question now is: Wich of the two values is the exact one?

Cheers, Freddy


Using the ESPG website you should be able to locate a local system that will give both graphic quality and measurement usability.Spatial reference

EPSG dot Org

Futher discussion on lat,lon should follow now...Lat Lon wiki

I have given the example that you take a basketball and hold a dowel (stick) pointing straight to the center (of the basketball). when you imagine the stick crossing through the center of the earth and through the intersection of equator/greenwich mean median that would be lat 0, lon 0. Moving the dowel to the east or west increases the angle from 0,0 either by positive numbers or in the case of the western hemisphere by - or negative numbers -100 lon, 52 lat. this is 100 degrees measured from the center of the earth west of greenwich mean median, and 52 degrees north of the equator (median). Take a basketball and a rod and do it yourself. It is really helpful to understand lat lon.

alt text

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