Experience in application of hand-held GPS in projects in Ethiopia
Salvat, Jerome
The objective of this paper is to evaluate the relevance of the use of hand-held GPS for cadastral purpose or, to know if hand-held GPS are accurate enough for cadastral mapping. The first step is therefore to estimate what the minimum requested accuracy level for cadastral surveying purpose should be. The first section of this paper introduces few facts about the GPS technology and its accuracy. The second part describes more precisely the hand-held GPS instrument based both on our own experience as professional land surveyors and on measurements conducted for this particular assignment. Based on the findings, a brief discussion is outlined to determine whether the use of hand-held GPS is suitable for a cadastral survey, and if not, which other means could be proposed as an alternative. Based on our experience both in Europe and Ethiopia, existing cadastral maps are not always very accurate. This is mainly due to the fact that a cadastral work is often a mix of different maps, such as title deed, cadastral map, master plan, etc., all prepared at different times with different accuracies. The result is that when one has to reconcile these documents, for a land sale or new land division for instance, adjustment is always necessary. In our opinion, such discrepancies should always be less than one meter. Indeed, beyond that figure, conflicts can be such that the buyer and seller might not be in a position to reach to a mutual and fair agreement. Additionally, cumulative errors of that range, on adjacent plots, will considerably affect the quality of the general cadastral map, finally making it completely inadequate. Consequently, a measurement method that will not afford at least better than one-meter accuracy, should not be considered as relevant in a cadastral survey.
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