# Meeting ALTA/NSPS Minimum Positional Precision Standards

The current MINIMUM STANDARD DETAIL REQUIREMENTS FOR ALTA/NSPS LAND TITLE SURVEYS (Effective 02/23/2016) specify that :

*“The maximum allowable Relative Positional Precision for an ALTA/NSPS Land Title Survey is 2 cm (0.07 feet) plus 50 parts per million (based on the direct distance between the two corners being tested).”*and it defines Relative Positional Precision as:

*“the length of the semi-major axis, expressed in feet or meters, of the error ellipse representing the uncertainty due to random errors in measurements in the location of the monument, or witness, marking any corner of the surveyed property relative to the monument, or witness, marking any other corner of the surveyed property at the 95 percent confidence level.”*So, how does one compute relative positional precision? The ALTA specification states that:

“Relative Positional Precision is estimated by the results of a correctly weighted least squares adjustment of the survey.”

If you are independently determining the coordinates of two points by some method such as, say, RTK, wanting to be certain that the length of the semi-major axis is no more than 2cm at the 95%-confidence level, and are using some positioning technique in a way such that the uncertainties of a pair of points are equal, then what is the maximum uncertainty that is allowable for the position of each point?

The answer is that

**a DRMS uncertainty greater than 7mm for the points will fail the ALTA/NSPS specification**for their relative positional precision when they are close enough together that the magnitude of 50ppm x D is neglible, where D = Distance between points. By DRMS, what is meant is that the uncertainty is such that the coordinates of the point as determined by survey have a roughly 68% likelihood of being within a distance of less than 7mm of the actual position of the point.In other words, to be certain that one is meeting the ALTA/NSPS specification, the points need to be positioned with an RMS uncertainty significantly less than +/-7mm. If you are using a positioning technique that on a good day, under ideal conditions, just barely has an RMS uncertainty of 7mm, then the odds are very good that the results are substandard.

The cure is simple.

**When the points positioned are close togther, use a better positioning technique than one that only delivers 7mm DRMS**. A total station should be able to easily do this or you can get several repeat measurements of the points positioned by the process with +/-7mm uncertainty, if the repeats actually do give a better answer instead of just one with the same errors as the first.

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