Spatial - Ed.com

NOAA Datums Videos 2014

Differential correction with NA2011 and the newest epoch of NAD83

See our and Datums Basics and GPS Tutorials for the long-awaited results of months of rigorous testing and peer review

Trimble Positions software has replaced Trimble GPS Analyst in ArcGIS 10.1

ALASKA GPS and GIS users - check out Joel Cusick's Datum and Projections presentation Feb 2013

ArcGIS 10.1 - New and Cool improvements see Presentation from Melita Kennedy, ESRI : Projections and Datums Dec 2012

Home GPS Basics Accuracy Defined
Accuracy Defined PDF Print E-mail

accuracy_defined

Accuracy specifications can be expressed in many ways

Circular Error of Probability (CEP)

CEP is based on a 50% confidence level. If 100 positions are collected at one point, 50% of them will be within the stated accuracy and 50% will be outside the stated accuracy.

Root Mean Square (RMS) error

RMS error is based on a 63 - 68% confidence level. If 100 positions are collected at one point, 63 - 68 of them will be within the stated accuracy, and 32 - 37 will be outside the stated accuracy.


2D Root Mean Square (2D RMS) error

Two Degree Root Mean Square (2D RMS) error is based on a 95% confidence level. If 100 positions are collected at one point, 95 of the positions will be within the stated accuracy, and 5 will be outside the stated accuracy.


Vertical accuracy

Depending on the geometry of the satellite constellation, the vertical accuracy of any GPS position may be from sub-meter to 3 times larger than the horizontal accuracy. The vertical component is difficult to calculate because SVs have a limited perspective in which to measure height. If the receiver could use signals from underneath it, the vertical component could be accurately measured, but the Earth blocks these signals.

Accuracy versus precision

accuracy_image

Accuracy describes how closely the calculated value compares to the known value. Precision is the measure of repeatability.