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Key Things to Know About Our Georeferencing
- Our coordinates are decimal degrees and datum is WGS84Georeferences include coordinates (latitude and longitude) in decimal degrees (WGS84 datum) and an error radius measured in meters that together define a circle. All serious users should understand that the true location of the occurrence may occur anywhere within the circle.
- We edited locality descriptions to correct and normalize spelling (using standard place names), and improved syntax of locality descriptions.
- For all georeferences we estimated spatial error defined as a radius starting at the coordinates, measured in meters. Our georeferences are thus circles. Just because we put a dot on a map does not mean that dot falls exactly where the specimen was collected. All serious users should review our methods and understand that the true location of the collection may occur anywhere within the error radius.Our notes indicate why locations could not be georeferenced and describe any decisions made about coodinates and error.
- Generally, georeferences were determined without consideration of taxa in order to ensure biases did not come into play. However, some georeferences were adjusted later based on distributional or habitat information when consensus could be made between at least two staff.
- Georeferences are continually refined based on new information. Comments from FoTX users are important for improving our georeferences.
- Once the data were georeferenced, we were able to use a Geographic Information System to populate our database with categorical variables such as county, river drainage, USGS hydrologic unit codes, and many more. Records that could not, or were not be georeferenced thus lack this step, lack this step. These data were extracted from the coordinates, regardless of error, so it is possible for categorical variables to be incorrectly assigned. Since our query interface allows for querying among these categorical spatial parameters there can be some confustion. For example, a location with coordinates in county A and an error that extends into county B will only be pulled when county A is designated in the query. Even though the overlapping georeferenced error estimate means that this occurrence could likely occur in county B, it cannot be found by querying for county B.
- Some records could not be georeferenced. This usually happens when there is an internal conflict in a locality description or a named place cannot be found.
- Fish obviously are limited to water, but not all coordinates in our database fall on water features. Users should remember that some locality descriptions are vague and do not allow precise placement of points.
- Georeferences are continually refined based on new information and comments from FoTX users are important for improving our georeferences.
Georeferencing Details
Initially we pursued various methods for georeferencing the records contained in the database. Among options were automated programs such as GeoLocate which (at the time) did not provide estimates of error and for which we were not satisfied with the placement of some points. Furthermore, it required substantial prior editing of locality descriptions to function optimally. Our Track 1 and 2 datasets (Verions 1.0 and 2.0 of the FoTX project) were georeferenced semi-manually using protocols outlined in various other large-scale georeferencing projects (HerpNET; MaNIS) and published documents (Wieczorek et. al., 2004). The methodology described in those documents defines a point with coordinates and assigns an error radius that defines a circle, throughout which the true location is equally probable to occur. In accordance with those protocols, an online error calculator was used to determine error radii, but coordinates were determined manually, in NAD27 datum (converted later to WGS84), using Terrain Navigator (Maptech) loaded with 1/24,000 and 1/100000 US Geographical Survey (USGS) topographic maps. Only points that met the criteria of having locality descriptions that were clearly described, without internal conflicts, and located within the project's geographic scope (i.e. located on the Texas mainland, bays, estuaries, or inland side of the barrier island's surf side) were georeferenced (although for Version 3.0 the scope was increased). In some instances, assumptions were made about misspellings or other intended meaning in the verbatim locality descriptions. For some instances in which a location could not be georeferenced we contacted the collector or examined original field notes if available for more information. Notes and assumptions are described in comment fields associated with each record and displayed on the website.
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