The New ASPRS Positional Accuracy Standards for Digital Geospatial Data
In November of 2014, the American Society of Photogrammetry and Remote Sensing (ASPRS) published the first ever mapping accuracy standards that are solely designed for today's digital geospatial data. Motivation Behind the New Standard is:
Legacy map accuracy standards, such as the ASPRS 1990 standard and the National Map Accuracy Standards (NMAS) of 1947, are outdated (over 30 years since ASPRS 1990 was written).
Many of the data acquisition and mapping technologies that these standards were based on are no longer used.
More recent advances in mapping technologies can now produce better quality and higher accuracy geospatial products and maps.
Legacy map accuracy standards were designed to deal with plotted or drawn maps as the only medium to represent geospatial data.
Within the past two decades (during the transition period between the hardcopy and softcopy mapping environments), most standard measures for relating GSD and map scale to the final mapping accuracy were inherited from photogrammetric practices using scanned film.
New mapping processes and methodologies have become much more sophisticated with advances in technology and advances in our knowledge of mapping processes and mathematical modeling.
Mapping accuracy can no longer be associated with camera geometry and flying altitude alone (focal length, xp, yp, B/H ratio, etc.).
- New map accuracy is influenced by many factors such as:
- the quality of camera calibration parameters;
- quality and size of a Charged Coupled Device (CCD) used in the digital camera CCD array;
- amount of imagery overlaps;
- quality of parallax determination or photo measurements;
- quality of the GPS signal;
- quality and density of ground controls;
- quality of the aerial triangulation solution;
- capability of the processing software to handle GPS drift and shift;
- capability of the processing software to handle camera self-calibration,
- the digital terrain model used for the production of orthoimagery.
These factors can vary widely from project to project, depending on the sensor used and the specific methodology. For these reasons, existing accuracy measures based on map scale, film scale, GSD, c-factor and scanning resolution no longer apply to current geospatial mapping practices.
- Elevation products from the new technologies and active sensors such as lidar, UAS, and IFSAR are not considered by the legacy mapping standards. New accuracy standards are needed to address elevation products derived from these technologies.
The New Standard Highlights
- Sensor agnostic, data driven: Positional Accuracy Thresholds which are independent of published GSD, map scale or contour interval
- It is All Metric!
- Unlimited Horizontal Accuracy Classes:
- Added additional Accuracy Measures
- Aerial triangulation accuracy,
- Ground controls accuracy,
- Orthoimagery seam lines accuracy,
- Lidar relative swath-to-swath accuracy,
- Recommended minimum Nominal Pulse Density (NPD)
- Horizontal accuracy of elevation data,
- Delineation of low confidence areas for elevation data
- Required number and spatial distribution of QA/QC check points based on project area
Advantage of Specifying the New ASPRS Positional Accuracy Standards for Digital Geospatial Data for a Project
Users of the new standards do not have to specify accuracy details for the intermediate processes in products generation. The user needs to specify the final deliverable product accuracy and the new standards will set up all accuracy specifications for intermediate processes, such as ground survey, aerial triangulation, etc., involved in the production of the final product. Figure 5 illustrates such a concept.