3

I am processing XML documents containing geographic information, and even though they are coming from the same source they are using different coordinates units. The first one is a regular latitude/longitude:

<linearExtension>
    <linearLineStringExtension>
        <gmlLineString srsName="EPSG:4326">
            <coordinates>12.13538573628832 60.10408278104808</coordinates>
        </gmlLineString>
    </linearLineStringExtension>
</linearExtension>

The second one is different:

<predefinedLocationContainer id="100193" type="PredefinedLocation" version="1">
    <predefinedLocationName>
        <values>
            <value lang="no">Tangenkrysset (55) - Dørdal</value>
        </values>
    </predefinedLocationName>
    <location type="Linear">
        <linearExtension>
            <linearLineStringExtension>
                <gmlLineString srsName="http://www.opengis.net/gml/srs/epsg.xml#32633">
                    <coordinates>180418 6548885, 180291 6548826, 180291 6548826, 180285 6548823, 180218 6548784, 180139 6548707, 180139 6548707, 180078 6548653, 180078 6548653, 179986 6548570
                </coordinates>
                </gmlLineString>
            </linearLineStringExtension>
        </linearExtension>
    </location>
</predefinedLocationContainer>

Are they absolute coordinates? If so, in what system? And if they are relative then in addition to the question of their unit system I should know the reference point which is not present here.

12

There are thousands of coordinate systems so it can be hard to tell which one is being used by just looking at the number of digits. However, 6-7 digits looks a lot like UTM coordinates, which are very widely used. In your example that is the case, indeed.

You can find this out by looking closely at the following line:

<gmlLineString srsName="http://www.opengis.net/gml/srs/epsg.xml#32633">

Coordinate systems are referred to by a specific code: the EPSG code. For example, in your first snippet you are using EPSG:4326, or in other words WGS 84, which uses degrees as units (latitude and longitude values, more specifically).

In your second example, it is not as ovious, but the EPSG code is still ‘hidden’ in the string. You can see that EPSG:32633 is being used, which you can simply put into Google and find out very quickly which coordinate systems that is. It is WGS 84 / UTM zone 33N.

Generally, I would recommend spatialreference.org to look up EPSG codes. In your example you would get this page, which gives you a lot of other useful information.

Also note: UTM units are always meters.

| improve this answer | |
  • 1
    UTM units are not always meters. Any projected coordinate system can have the usual/native/whatever units converted to a different unit. Meters is a good default assumption, and usually if it isn't the projection name will have the units in it (ie "UTM Zone 18N in feet"), but it should always be verified. – Chris W Mar 8 '15 at 20:47
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    @ChrisW I am pretty sure that all UTM definitions use meters, for the simple reason that feet are not universal. EPSG does not list any either. There are local CRS that have feet as units, like some State Plane CRS. But these are not named UTM. – AndreJ Mar 9 '15 at 13:46
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    @AndreJ You're far more an expert than I am, and those I've run across may be just localized versions created by someone or some agency and not 'official' beyond that. Spatialreference lists a couple, but not under EPSG and of course anyone could have uploaded them. I note the ones I most know of aren't called UTM, as you say - they're named BLM. And yes, state plane often comes in at least two flavors of feet and meters. – Chris W Mar 9 '15 at 17:41
  • Yes, BLM is a US-feet adaption of UTM, see mappingcenter.esri.com/index.cfm?fa=ask.answers&q=983 – AndreJ Mar 9 '15 at 17:57

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