anbul 2004 
ration. 
  
Integrated approach to predict confidence of GPS measurement 
Massoud Sharif à, A. Stein @, Ernst M. Schetselaar b 
À Department of Earth Observation Sciences, sharif@itc.nl, stein(@itc.nl, 
  
International Institute for Geo-Information Science and Earth Observation (ITC), PO Box 6, 7500 AA Enschede, The Netherlands. 
Commision III, WG IIUI/8 
KEY WORDS: Geodesy, surveying, Acquisition, analysis, Transformation, GPS, Accuracy, performance 
ABSTRACT 
Code measurement hand-held GPS serves a fast growing user community involved in navigation and data collection for field data 
collection, tourism, various public services and commercial applications. Most handheld GPS code receivers, however, do not 
provide instantaneous and intuitive information on the positional error during measurement, which may be important to compromise 
data accuracy against time efficiency in daily field practice. Herein we describe an experiment on how to assess the positional error 
of handheld code receivers and discuss a method that improves on the reporting of the positional error to the user in the field when 
reference points are available. In the experiment, the x and y coordinates of 100 reference points were used to assess the positional 
error of a handheld GPS code receiver. The coordinates of the points were computed from DGPS carrier phase measurements, and 
assumed to be error free. The error distribution in x and y for 1, 3, 5 and 10 minute averaging periods were computed to generate a 
figure of merit that is intuitive and informative to a non-expert GPS user. Results obtained are well within the range of the theoretical 
accuracy that can be expected for single measurements code receivers. 
INTRODUCTION 
Low cost handheld GPS receivers have become increasingly 
popular to a variety of users providing position, velocity and 
navigation information under all weather conditions. Most of 
these users need to know their position relative to a mapping 
base or real world topographic feature in real time in order to 
navigate, find, record or map geographic features. The users of 
a handheld GPS receiver, however, have only limited means to 
assess the accuracy of the GPS measurements in the field and 
hence cannot instantaneously obtain reliable information on the 
quality of GPS measurements and its variability. 
The manuals of most GPS units provide a rough theoretical 
estimate of the horizontal and vertical accuracies. In addition, 
many units report statements of probable error, such as 
estimated position error (EPE) and figure of merit (FOM). The 
EPE is a scalar indicating the precision of the receiver based on 
the deviation of the measurements from the mean of the 
measurement. The FOM is a more mysterious quantity that uses 
in addition to the statistics of the measurements, additional 
information held confidential by the manufacturers of the code 
receivers, but which probably in addition to statistics on the 
GPS measurements uses either a theoretical or empirical error 
model to predict the accuracy of a code receiver. 
Hence, users of single handheld GPS units do not have 
instantaneous access to explicit accuracy reports of the GPS 
measurements. This is, besides the lack of explanation on how 
the accuracy indicators are computed, also due to the lack of 
confidence intervals given in the displayed EPE reports. For 
example it is often not known whether the EPE is based on a 50 
627 
% confidence interval, the route mean square error (68 % 
interval) or any other confidence interval. Given the above, the 
non-expert GPS user has no effective means to obtain insight in 
the accuracy of the handheld GPS equipment using it at a 
particular location and time. To overcome this limitation, the 
user needs to be able to conduct field experiments to assess the 
accuracy of the GPS unit in the area of interest and generate 
accuracy reports that are unambiguous and intuitive to 
understand. Ultimately these reports may, together with the 
statistics of the GPS measurements acquired at a given location 
and time, provide a basis for empirical predictions of the 
accuracy (e.g. FOM) of a particular instrument when no 
reference points are available. 
In this paper we discuss an experiment that can be used to 
assess and generate reports of the accuracy of single handheld 
GPS units. The focus of this paper is on the accuracy of the 
horizontal position information, although accuracy assessment 
has a bearing on time, horizontal and vertical position, and 
velocity as well. Our analysis refers to the assessment to what 
extent the measured position of a particular point on the surface 
of the Earth conforms with respect to the true position of that 
point. 
MATERIALS AND METHODS. 
When a handheld GPS code receiver is used for positioning, 
one can get a rough indication about the accuracy by referring 
to the theoretical figure of the GPS error model, as presented in 
table 1.