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Georeferencing (converting locality names to coordinates)

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Introduction

Georeferencing is the process of assigning coordinates (e.g. in latitude and longitude) to a text-based description of a locality. There are several methods to do this:

  • Automatically - via a gazetteer (look-up table, or service, which cross matches locality names to pre-defined coordinates)
  • Manually - via a printed map, or digital representation of the same.

The georeferencing comparison page presents a comparison of some of these options.

Gazetteers

Here are some on-line gazetteers:

it is important to remember that both the accuracy and the precision of gazetteer entries can vary. Accuracy and precision are actually independent; for example if the true position is located at 14.13° N, then 14.1° is accurate but not completely precise, 14.77° is precise but not accurate.

Many gazetteer entries are quoted only to the nearest minute of latitude or longitude (around 1.3 km, i.e. a precision of +/- 0.7 km). Entries for the same locality can also vary from one source to the next, for example Hobart (Tasmania) appears as 42° 52' S, 147° 19' E in the online Gazetteer of Australia, 42° 55' S, 147° 20' E in the online ADL Gazetteer, and 42° 50' S, 147° 15' E in the printed Atlas of Australia (Readers Digest 1994).

Quoted coordinates vs. real extents

Another problem with gazetteers is that, most frequently, the area they are describing has some extent (for example a town, river or bay) however the coordinates are typically quoted for a single point which is (presumably) considered to represent the centre of the feature. Hence for the highest accuracy, manual georeferencing from large scale local maps (e.g. 1:50 000, 1:25 000) is preferable.

Printed / digital maps

Currently, for Australia these mapsheet series can be purchased as digital files or hard copies, plus in the case of the 1:50 000 series, are available online (reprojected to Plate Carree format) via the Earthetc.com website.

For an index of mapsheets available for purchase from the Australian national mapping agency, Geoscience Australia, see this link:

Newer interactive tools such as Nasa's World Wind and Google Earth also allow manual georeferencing via cross-hairs or a latitude-longitude grid once the user has zoomed to the relevant location, provided that this is recognisable. These two products also have a gazetteer function built in (i.e., go to a named location), though of the two, the GoogleEarth product seems less reliable for the Australian region than the NASA one, at least at the present time, and in both cases the named features are largely confined to populated places.

The following example compares some of the above georeferencing options for a named coastal locality - Pipe Clay Lagoon in Tasmania, Australia:

Offsets

Sometimes localities are quoted as offsets from a named feature, e.g. "3 miles south of Point Hicks". In these cases, it will be necessary to convert miles (or km or nautical miles) to degrees using a resource such as the following:

Uncertainty

Sometimes a locality is not known precisely, e.g. "Great Oyster Bay, Tasmania" or "Bass Strait" occupy considerable areas and the precise location of the collecting event within them was not recorded. For these purposes, OBIS provides a field "Coordinates precision" for which the definition is: "An estimate of how tightly the locality was specified in the Latitude and Longitude fields; expressed as a distance, in meters, that corresponds to a radius around the latitude-longitude coordinates." (A similar field exists to express precision for start/end coordinates). In the Bass Strait example above, therefore, it would be appropriate to indicate the approximate centre of Bass Strait in decimal degrees (e.g. latitude -40, longitude 146.5) together with a "Coordinates precision" value of 170000 (since the overall length [maximum dimension] of Bass Strait is around 340 km or 340000 meters).

Datums

OBIS follows established standards for data exchange and requests latitude and longitude values to be expressed in the WGS84 datum. For practical purposes this is very close (typically within 1 metre concurrence) to the GDA94 datum used on current Australian topographic maps. Previously used systems in Australia such as AGD66 and AGD84 will show items shifted in the order of 200m from their current (GDA94) coordinate-based positions. If changes of this magnitude are significant for your data, or if older and possibly non-standard datums were used at the time the coordinates were quoted, then it may be useful to visit the Geocentric Datum of Australia FAQ maintained at ICSM Australia, however for many (?most) cases this discrepancy may be below the "noise" level since it represents only around 0.0025 decimal degrees, or 0.15 minutes in degrees/minutes notation.

More information

The (U.S.) BioGeomancer project is currently involved in the development of a number of methods and online tools for georeferencing. Their website (www.biogeomancer.org/) is well worth visiting for further information on many aspects of georeferencing.

Created by admin
Last modified 18-12-2012
 

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