PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
January 2015
15
I was among the attendees of your session on the new
ASPRS Map Accuracy Standards during the last ASPRS
fall conference held in Denver in November 2014. Could
you please elaborate more on the new standards, its
similarity with the previous standards and how to use it?
Anonymous
Dr. Abdullah:
The topic of understanding the new standards,
referred to as the “ASPRS Positional Accuracy Standards for
Digital Geospatial Data”, is very important for all users of the
new standards during this transition period. Therefore, I will
dedicate more than one article to introduce the new standards,
highlight its similarity with the legacy standards of the ASPRS
1990 and the National Map Accuracy Standards (NMAS), and
provide examples on how to relate the new standards to the
legacy standards. The new standards were approved by the
ASPRS board during their meeting in Denver on November
17, 2014 and became the official new ASPRS map accuracy
standards replacing the old standards of 1990.
As stated in the standards, the new standards are published
to meet the dire needs of new era for the geospatial community.
Such an era is characterized by rapid advancement in the field
of geospatial data collection and data production. Among the
main reasons that led to the development of the new standards
are the following:
1.
Legacy map accuracy standards, the ASPRS 1990
standards and the NMAS of 1947, are outdated. (over 30
years since the ASPRS1990 standards were published)
2.
Many of the data acquisition and mapping technologies
that legacy standards were based on are no longer used
in today’s mapping process.
3.
More recent advances in mapping technologies and
methods enabled us to produce better quality and higher
accuracy geospatial products and maps.
4.
Legacy map accuracy standards were designed to deal
with plotted or drawn maps as the only medium to
represent geospatial data. Today’s digital mapping
workflow requires different accuracy measures that are
more suitable for the digital products.
5.
Within the past two decades (during the transition period
from hardcopy to softcopy mapping environments), most
measures for relating image Ground Sampling Distance
(GSD), map scale, and contours interval to the final
product accuracy were based on film scanning practices
which were established prior to the introduction of the
first digital camera . Such practices and measures are
no longer suitable for the products from digital sensors.
6.
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. Such sophistication resulted in
more accurate mapping products.
7.
Mapping accuracy can no longer be associated with the
camera geometry and flying altitude alone (focal length,
xp, yp, B/H ratio, etc.). Accuracy of newmapping products
is influenced by many other 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;
•
quality of parallax determination or photo measure-
ments;
•
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,
•
quality of 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 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.
8.
Elevation products from modern active sensors such as
LiDAR and IFSAR were not considered by the legacy
mapping standards as it did not exist then. Therefore,
new accuracy standards are needed to address elevation
products derived from these technologies.
More information on the motivation behind publishing the
new standards can be found in annex A of the new standards
document
