Horizontal Positional Accuracy Assessment of an Orthopohoto: A case of Addis Ababa City
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Date
2019-06
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Addis Ababa University
Abstract
This study tries to examine horizontal positional accuracy of orthophotoof Addis Ababa city
using in-situ Global Positioning System (GPS) data. GPS data is processed using different
processing systems like spectrum survey, Leica GeoOfice(LGO) and also online processing
systems AUSPOS and OPUS in a least square sense with a tie to local and regional GPS
reference stations to predict horizontal coordinates at twenty checkpoints to assess the accuracy
of orthophoto. The accuracy of horizontal coordinates of the checkpoints derived from
orthophoto was evaluated by comparing it with the corresponding coordinates measured directly
from in-situ GPS data. When we compared the orthophoto with that of online processing AUSPS
it is in a good agreement with GPS data observed at twenty checkpoints with a root mean square
error of 9.3 cm in x- and 9.9 cm in y-coordinates. On the other hand, the horizontal accuracy of
orthophoto is determined at 3.19 cm as it compared with twenty check points with 95%
confidence level. Whereas, when the GPS data are processed using sokkia spectrum with a tie to
local GPS reference station the horizontal coordinates of the orthophoto are in agreement with
in-situ GPS coordinates at an accuracy of 7.2 cm and 21.39 cm in x and y-directions
respectively, The finding of the result illustrates that the horizontal accuracy of orthophoto is
determined at 8.81 cm with 95% confidence level, while the GPS data is processed with a tie to
local GPS network. Therefore, it can be concluded that, the accuracy of the orthophoto is within
the national standard of error budget (~ 25cm).
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Keywords
AUSPOS, OPUS, orthophoto, GPS data, horizontal accuracy