The OSNI large-scale mapping has been captured over a period of many years, beginning with the first surveys in the 1830s. During this time, survey instruments and technology have continuously developed to provide improved precision for measurements and positions.
New surveying methods such as Total Stations with Electronic Distance Measuring (EDM), pen computers and especially satellite positioning systems, such as the Global Positioning System (GPS), are now used in the data capture of our large-scale digital mapping and have improved positional accuracy. The positions obtained in more recent surveys that have utilised these technologies may not correspond with those determined using previous survey methods.
The increasing use of more sophisticated data capture equipment and the requirement to combine digital datasets within a Geographical Information System (GIS), makes the absolute positioning of spatial data more critical. This changes the emphasis on the type of accuracy required for OSNI data from high relative accuracy between points to high absolute accuracy for all points.
OSNI aims to ensure the absolute accuracy of our surveyed map detail to the following respective accuracies:
TYPE OF SURVEY
RMSE
1:1,250 resurvey 0.4m
1:2,500 resurvey 0.8m
1:2,500 overhaul 2.5m
1:10,000 overhaul 5.6m
In recognizing the changing customer needs we have now begun to quantify the current positional accuracy of our data. This will help in the development of a formal long-term policy. Building on the sample findings this review will consider what will be required to improve the absolute position of all data, the timeframe, and method to complete the upgrade. Once these details are established a customer consultation will be undertaken, meantime regular updates will be made available through the OSNI web site as the project progresses.
For further information please contact:
Ken Stewart
Tel: 028 90388492
E: ken.stewart@osni.gov.uk
Relative Accuracy is the difference in the distance measured between two points on a map and the true distance between these same two points, which is measured using conventional surveying methods.
Absolute accuracy is the distance between the position of a point on a Large Scale Map and its True position within the reference frame. In assessing Positional Improvement, the true position is measured using GPS.
RMSE is the root mean square vector error for points of hard detail based on their Irish Grid position, after the removal of any detectable systematic error, and is indicative only.
