How to establish map projections to facilitate the use of GPS on ground-based surveys

Abstract

[Abstract]: With the increased use of RTK GPS, local ground-based coordinate systems are increasingly being used to represent ground distances on plans. Through their use, the incorrect representation of ground distances on plans can be minimised. However, many of the users of such systems do not know the limitations of these systems. This dissertation developed and analysed two local ground-based coordinate systems based on Transverse Mercator and Tangent Plane projections. The method involved establishing local ground-based coordinate systems at an average project height at which grid distances approximately equalled ground distances over small areas. The testing focused on distance and angular errors caused purely by the process of projection. Other site dependent variables have also been assessed, including the effect of site height above and below projection level and the effect that the longitude of the site has on the distance accuracy of a site. It was concluded that the major limiting factor, when using local ground-based coordinate systems, is the error in grid distances when compared to measured ground distances. The results obtained show a variation in error distribution between the coordinate systems, depending on the method of projection used. The results illustrate a number of accurate areas within which a number of defined measurement accuracies and magnitudes are not exceeded. The limits of the systems were found to be approximately 3km east/west in a Transverse Mercator projection and 19km in any direction in a plane system, from the central point of the site, before RTK GPS measurement accuracy is exceeded by projection distortion. The need for quantification of errors in local ground-based coordinate systems is significant as they are used to produce plan distances when using RTK GPS. If used outside the limits defined in this dissertation, errors will occur in the plan distances resulting from the use of such systems.