Geography Reference
In-Depth Information
Accuracy of Georeferencing
According to a recent article in the International Journal of Geographic Information Science ,
six steps are required for determining georeferences to places lacking precise coordinate
references—for example, 6 miles NW of Timmons, NV. Knowing these six steps is important
because many places with descriptive data lack accurate locational references; however,
coordinates must be used for storing the descriptions as geographic information. The point-
radius method summarized here provides consistent and accurate interpretations of locality
descriptions and identifies potential sources of uncertainty.
Step 1: Classify the locality description . The quality of the description should be assessed
and classified. Only somewhat accurately described localities should be georeferenced.
Step 2: Determine coordinates . Coordinates can be retrieved from gazetteers, geographic
name databases, maps, or from local descriptions with coordinates—for example, field
notes with GPS coordinates. The numerical precision of coordinates should be pre-
served during processing to minimize the propagation of error. Next, identify named
places and determine their extents . Every named place has an extent. This should be
determined in the same manner as the coordinates of the locality. Most named places
have a geographic center (courthouse, church) which should be used as the origin of
circle defining the extent. Then, determine offsets . Many localities are located by their
relationship to another place—for example, 6 miles NW of Timmons. The direction from
the place can usually be inferred, considering environmental constraints and additional
information in the description. Supplementary sources are helpful.
Step 3: Calculate uncertainties . In this article, Wieczorek et al, consider six sources of
uncertainty:
1. Extent of the locality. The maximum extent of two places in the locality is the maxi-
mum uncertainty.
2. Unknown datum. The differences can be as large as 500 m between NAD27 and
NAD83. Theoretically the difference could be as large as 3,552 m.
3. Imprecision in distance measurements. Treat the decimal portion of distance
measurements as a fraction and multiply the distance measurement by this frac-
tion. Multiples of powers of 10 should be multiplied by 0.5 to that power of 10.
4. Imprecision in direction measurements. Translate cardinal directions to their
degree equivalents, using half of that degree equivalent as the uncertainty.
5. Imprecision in coordinate measurements. Consider latitude and longitude error.
uncertainty = lat _ error 2 + long _ error 2
6. Map scale. Take the error of a map to be 1mm. For example, the uncertainty for
a map of scale 1:500,000 is 500 m.
Step 4: Calculate combined uncertainties. The uncertainties without directional imprecision
and combined distance and direction uncertainties should be calculated following map
accuracy guidelines for the maps used. Distance uncertainties should take directional
imprecision into account.
(cont.)
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