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I have data that spreads across two UTM Zones GDA/MGA94 UTM Zone 55 and 56. I have layers such as a cadastre, roads, pipelines etc where I want to store measurements like lengths and areas in metres or some kind of metric notation, not degrees!

Whats the best way to manage this in PostGIS (i'm using PostGreSQL 8.4 PostGIS 1.5)? Should I be storing my data as GDA94 Geographic coords and using some sort of work around to calculate the measurements I require? Or is there another way to handle this?

I'm keen to hear how other people address this situation.

Thanks Ando

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5 Answers

up vote 9 down vote accepted

Based on the information provided I would be looking at using a different projection which is not tied to the UTM Zones, like Geoscience Australia's Lamberts Conformal Conic Projection. This will have some distortion effects but they will be minimal. The distance units are then in metres. It should also get through the issue of the state crossing as all agencies are meant to be able to use this projection as GA is the primary mapping body in the country. Check with your requirements as this may be an option.

The projection is available for download if not provided from GA's website.

have fun, CDB

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just have to be careful using Lamberts Conformal Conic when you are interested particularly in areas, it is not an equal - area projection and thus introduces errors in area calculation. –  Kelso Nov 30 '10 at 22:10
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Have you tried using the geography data type. That stores in long lat and returns measurements in meters. You can then use a combination of CAST to geometry and transform for display if you need to project for displaying the data.

http://www.postgis.org/documentation/manual-1.5SVN/ch04.html#PostGIS_GeographyVSGeometry http://www.postgis.org/documentation/manual-1.5SVN/ch04.html#PostGIS_Geography

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Thanks I had seen the geography data type mentioned, I will read your links to find more out about it. I also have the PDF version of PostGIS in Action, it mentions that for localised areas the geography data type measurements tend to not be as precise as geometry measurements. –  Ando Nov 23 '10 at 20:25
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Is there not a local projection for the country/region you are working in? If so that would be the best one to use.

Edit based on comments:

Looking on http://spatialreference.org for Australia indicates that EPSG:3112 would work for you or EPSG:3577, depending on if you need equal area or conformal shapes.

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There is. However we also cross two states, and they both have a Lambert projection that they push for state wide mapping and data storage. –  Ando Nov 23 '10 at 20:40
    
NSW and VIC? There's no easy way to do this without adopting one or the other projection (or your own) and just transforming between different data sources as required. –  mdsumner Nov 23 '10 at 23:47
    
Yep NSW and VIC. I want to avoid creating my own projection if I can. –  Ando Nov 24 '10 at 0:14
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Pick the UTM that most of your data falls in, and use it for all data. UTM Zones have (a half degree I believe of) overlap for just this reason. Measurements won't be too far off in this overlap zone.

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This is actually not good advice. The distortion in a UTM projection increases the farther east/west you get from the central meridian. The reason the concept of zones were adapted in the first place is to establish a cutoff point at which that distortion becomes unacceptable. An Albers projection is a better compromise for data >6 degrees in extent (also see gis.stackexchange.com/questions/2769/…) –  matt wilkie May 18 '11 at 17:59
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That's a reasonable caution, @Matt, but we need to bear in mind that "acceptable" depends on the circumstances. For instance, that half degree can be extended many degrees up in the extreme latitudes with no ill effect. For a discussion of the distortion introduced by crossing UTM zone boundaries, please see the (more recent) thread at gis.stackexchange.com/questions/31701/…. –  whuber Aug 25 '12 at 2:55
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Since you deal with long linear features like pipelines, the best solution is to store the data in the good old 'WorldMercartor' projection (epsg 3395). Then you can obtain the large scale data. If you need something more precise for a given spot, then write a query to reproject that spot to a local projection to obtain precise data.

Remember that the Mercartor projection is famous becouse it preserves lines as lines (not arcs) over great distances.

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What do you mean "lines as lines (not arcs)"? It correctly represents loxodromes as straight lines, and it's famous for that because of ocean navigation by compass, and for being a simple projection to generate. –  mdsumner Nov 30 '10 at 22:26
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