Most potential users of the Spatial Calculator will immediately ask one question: How accurate are these calculated latitudes and longitudes? Clearly, the calculator cannot achieve the sub-meter accuracy level of GPS technology. But, neither does it require the significant investments of GPS. The following factors influence the accuracy of the Spatial Calculator's results.
The Base Map: The base map of latitudes and longitudes that is being used by the calculator is a landgrid database that belongs to Topographic Mapping Company (TMC) of Oklahoma City. TMC originally created this database by digitizing section lines from the USGS 7.5 minute (1:24,000) quadrangle maps for the State of Oklahoma. This scale allows an assumed accuracy within 40 feet.
The Datum: The USGS maps, and therefore the original landgrid database, were based on the North American Datum 27 (NAD27). We have recalculated the entire landgrid database to NAD83, so users have the choice of either datum. (NAD27 is based on an ellipsoid model of the earth with a fixed base point of Meade's Ranch, Kansas. NAD83 is based on a more recent ellipsoid model that was derived from satellite measurements of the earth with the correct equatorial and polar radii.) To maximize your map accuracy, you will want to match the datum for any other features you might be mapping along side these calculated points. If you are not sure which datum to use, choose NAD83.
The Quartering Process: It would be easy if the Public Land Survey System (PLSS) was a perfect grid of one-square-mile sections (5280 feet by 5280 feet) that we could partition into four equal quarter sections. Since it is not, we have implemented a quartering methodology as follows.
For regular or slightly irregular sections
(those that vary from one-square-mile by no more than 10%, or 528 feet), standard PLSS rules are followed:
The southeast (SE) quarter is always the standard size of 2640 by 2640 feet, and the remaining quarters get the "leftovers", whether those are more or less than the standard quarter section size.
By extension, the same standard is followed for the SE quarter of any given quarter, quarter-quarter, or quarter-quarter-quarter - the default sizes being 1320, 660, and 330 feet respectively.
In these cases, we consider the SE to be the "corner of origin" for the quartering process.
For very irregular sections
(those that vary from one-square- mile by more than 10%, or 528 feet), in Oklahoma the corner of origin varies by section. In some sections, the SE quarter is dominant, while in other sections the NE, SW or NW quarter might be dominant. We have reviewed the quarter-call practices for all of the very irregular sections in the state to identify a corner of origin for each.
When the section is less than a half mile wide or tall, then the quarter sections do not exist from the half that is not in the corner of origin. If you try to calculate a point from a quarter that does not exist according to these rules, no calculation will be attempted and an error message will be returned.
When the section is at least a half mile wide or tall, the "corner of origin" quarter is always the standard size of 2640 by 2640 feet, and the remaining quarters get the "leftovers", whether those are more or less than the standard quarter section size.
As with regular sections, the same standards are followed for the corner of origin quarter of any given quarter, quarter-quarter, or quarter-quarter-quarter - the default sizes being 1320, 660, and 330 feet respectively.
When the calculator processes a point from a very irregular section, it generates a warning flag to identify potential problem areas.
There are a few sections in the state for which the section shape is so unusual that we cannot designate a corner of origin, and we do not attempt to calculate any points within those sections. Here is a list of those extremely irregular sections.
Since this quartering process has so many complexities, there are probably
ways in which our methodology could be improved. We welcome user input that
would help us refine this quartering process. Please
contact us if you have any suggestions or questions.