TABLE 1 
ACCURACY COMPARISONS 
  
DATABASE BEST CASE ACCURACIES 
METRIC TEMPORAL FEATURE 
SPOT so” 1-2 Months Image 
(1) 
DLG 165/ 2-20 Years Select 
1:100,000 (2) Features 
(3) 
DLG 40 2-20 Years Select 
1:24,000 (4) Features 
(3) 
  
(2) Many are 10 years or more 
(3) PLSS Grid, Boundaries, 
Hypsography, 
Metric Accuracy 
The metric or positional accuracy relates to 
the matching of the features in the map 
graphic data sets to the actual locations on 
the surface of the earth, as defined by a 
standard reference system. For a high degree 
of metric accuracy the map graphic data must 
accurately represent the information on the 
ground for the purposes of measurement of 
distance and direction. To be considered a 
scaled representation of the surface of the 
earth an image, whether satellite or aerial 
photo, needs to be corrected, or rectified, 
due to sensor induced and ground surface 
distortions. Because the DLG data, at either 
Scale, is developed from photography, it too 
is influenced by this rectification process. 
It then goes through the additional process of 
being collected in vector form, as lines, 
points and polygons in the DLG data set. 
The metric accuracy of uncorrected SPOT 
imagery can be as great as 1000 meters, which 
would not be used to support GIS applications. 
However if multiple SPOT images are run 
through a rigorous simultaneous block 
triangulation solution and then orthorectified 
using existing or newly generated Digital 
Terrain Model (DTM) information, the image 
distortions caused by the sensor and the 
undulations of the earths surface can be 
corrected for a final image mosaic that 
achieves an accuracy that easily supports GIS 
applications for regional use, 
In addition, because the techniques used for 
the correction of the images involves using 
existing ground control, it is very easy to 
compute horizontal coordinates, (latitude, 
longitude) for any pixel in the digital image. 
The elevation can be computed from the DTM for 
the area. The metric accuracy then becomes a 
function of the pixel resolution, the control 
source used in the triangulation, and the 
accuracy of the DTM used in the 
orthorectification process. 
Orthorectified SPOT image mosaics with 
absolute horizontal accuracies of 50 feet are 
available today. The absolute horizontal 
accuracy associated to the existing 1:24,000 
USGS Quad sheets is 40 feet and the USGS 
1:100,000 DLG data has positional accuracies 
(can purchase as individual data or 
Many hard copies are 10 years or more, 
Will be used to generate the new 1:24,00 
684 
10 meter pixel image data/orthorectified and mosaicked 
Transportation, Hydrography, 
all) 
but new photography 
O DLG dataset 
of 165 feet. The credibility of the DLG 
positional accuracies is sometimes 
questionable due to the age of the source 
materials used to compile the information. 
The SPOT imagery, after the appropriate 
triangulation and orthorectification 
processes, has a significantly higher metric 
accuracy than the 1:100,000 DLG, and a metric 
accuracy that approaches that of the 1:24,000 
DLG to be developed in the future. 
Temporal Accuracy 
Temporal accuracy refers to the "timeliness" 
of the data. Is it out of date? How often is 
it updated? How difficult is the update 
process? The maintenance of map graphic data 
is often overlooked in building a GIS when 
indeed this is one of the most important 
issues in GIS operations. The development of 
digital data from 1:24,000 USGS quad sheets or 
using the USGS 1:100,000 DLG data the user 
should look closely at the temporal accuracy. 
Even when it is a newly developed digital data 
set, the temporal accuracy can be in question, 
due to the sources used. Many times the 
existing USGS data are no more recent than the 
late 1970's. The information has been photo 
revised in some areas. 
SPOT imagery can realistically be retrieved 
rectified, and used to generate new map 
graphic data within a one to two month window. 
After the development of a large area multiple 
image mosaic, updates can be done to specific 
subset areas within the data set, within one 
or two months. For example, those areas in 
which the user knows there has been 
significant change can be clipped out of the 
existing SPOT mosaic map graphic data set and 
the new image data can be used to replace it. 
The end result is new imagery within old 
imagery, all rectified and mosaicked with no 
image edges detectable. The one to two month 
update time is dependant on atmospheric 
conditions and acquisition priorities, but 
even if maintenance images are only available 
every 6 months it is far more recent than the 
5 or 10 year cycle commonly associated with 
aerial photo missions flown for map update 
purposes.