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iGage iG9 User Manual
OPUS: What is it?
OPUS (Online Positioning User Service) is a free service provided by the NGS (National Geodetic
Survey.)
From the NGS Website:
“This Online Positioning User Service (OPUS) provides simplified access to high
-accuracy National Spatial Reference
System (NSRS) coordinates. Upload a GPS data file collected with a survey-grade receiver and obtain an NSRS
position via email.
OPUS requires minimal user input and uses software which computes coordinates for NGS' Continuously Operating
Reference Station (CORS) network. The resulting positions are accurate and consistent with other National Spatial
Reference System users.”
Here are direct links to more detailed information:
http://geodesy.noaa.gov/INFO/OnePagers/OPUSOnePager.pdf
http://geodesy.noaa.gov/OPUS/about.jsp
One of the most important contributors to vertical accuracy computed by OPUS-Static is the length
of occupation: longer times are better.
If you are concerned about elevation, please remember that a 2-hour OPUS static observation has
an expected height accuracy of 2.5 cm. A 6-hour occupation has an expected accuracy of 1.5 cm.
Again, longer times are better. Your expectations should be tempered by this NGS graph:
OPUS-RS (Rapid Static)
Standard OPUS-Static sessions require 2-hour observations. OPUS-RS sessions can be as short as 15-
minutes.
However, OPUS-RS solutions are not available universally. In general, OPUS-RS requires:
3 (or more) CORS within 250 km of your site your site must be within 50 km of the polygon formed by the CORS sites
If you are working in Southwest Nevada, along the highline of Montana or in North or South Dakota,
OPUS-RS probably will not work and you will have to collect more than 2 hours of data for
submission to OPUS-STATIC!
Prior to collecting data for OPUS-RS check the latest status map to ensure that OPUS-RS will work.
The online OPUS-RS resource
http://geodesy.noaa.gov/OPUSI/Plots/Gmap/OPUSRS_sigmap.shtml
is updated routinely and reflects the probability that an occupation at a given location will be
successful and the expected accuracy for a 15-minute and 1-hour occupation.
