PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
September 2016
659
Q: Although we are using digital cameras with different
focal lengths and configurations and therefore different
flying altitudes to acquire imagery with a resolution
of 10 centimeters, my client insist on applying the
following rules in associating imagery resolution and
final products scales and accuracies:
1. Mapping Scale = 7*aerial photo scale
2. Contour Interval = flying height / 1,800 to 2,100
3. For imagery with resolution of 10 centimeters, we
have to provide the client with mapping products
with a map scale of 1:1000 and positional accuracy
of RMSE = +/- 20 centimeters in X, Y, or Z.
Is there a direct relationship between map scale and
GSD? If so, what is the appropriate map scale that
we can produce from imagery with resolution of 10
centimeters?
Michael Younan, Good Shepherd Engineering,
Bethlehem, Palestine
Dr. Abdullah:
Looking through the history of map making,
one can easily notice the influence of hardcopy media, i.e.
paper, that we used to publish and communicate all matters
related to the maps, including positional accuracy. Because
maps were only produced on paper, we needed to define map
scale so users could measure the physical distance on a paper
map and relate it to the actual distance on the surface of
the earth. Map scale dictated the film-to-map enlargement
ratio, flying altitude, map accuracy, etc. We used this
association of paper map scale and all other subjects related
to the map production for decades, and some users seem
confused and unsecured during the transition period to
digital imagery and digital map production workflow. The
current reality of the new digital workflow used in almost
all geospatial production processes makes it difficult to
connect to the legacy paper maps workflow. Digital imagery
and lidar produce digital maps and digital elevation data
that are no longer associated or defined by map scale or
contour interval. Digital imagery and other digital aerial
products should not be labeled or associated with map
scale, GSD, and/or contour interval. Map scale and contour
interval were created to deal with the limitations of the
mapping instruments and paper media at the time. They
have no room in the all-digital workflow we are using today
to produce digital geospatial products. Digital imagery
with certain resolution, i.e. GSD, can be used to create
maps with different GSD and accuracy. The new ASPRS
Positional Accuracy Standard for Digital Geospatial Data
(
PE&RS
, March 2015) is based on the latter understanding.
In the new standard, accuracy is expressed independently
from GSD, map scale or contour interval. The map can be
produced and labeled according to the users need and not
according to the imagery resolution or map scale. Today’s
digital workflow can achieve different end-user required
accuracy by applying different stringency in the workflow.
Users need to realize this new reality and start labeling
products accuracy without thinking of paper map scale or
contour interval. Your client criteria given in (1) above is
expressing a film-to-map enlargement ratio of seven. With
digital imagery, we do not use enlargement ratio. That was
valid when we projected the film on a paper to enlarge it
seven times to produce a paper map with a certain scale
that is seven times larger than the negative (film) scale.
Digital imagery is closely associated with the digital zoom
on the screen. It is not limited to any map scale as it varies
depending on the zoom level that the user selects. Scale for
each zoom level can only be determined by measuring the
physical distance on the computer or the tablet screen and
“We used this association of paper map scale and
all other subjects related to the map production
for decades, and some users seem confused and
unsecured during the transition period to digital
imagery and digital map production workflow”
Photogrammetric Engineering & Remote Sensing
Vol. 82, No. 9, September 2016, pp. 659–663.
0099-1112/16/659–663
© 2016 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.82.9.659
