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iGage
iG8 User Manual
above 10° elevation, the number of observations used
will be lower.
Make sure you use the left-hand column (NAD_83)
results, not the right-hand column (IGS08.)
Be careful with heights. Both ellipsoid and orthometric
heights are listed. The orthometric height is NAVD88
GPS derived and typically is the elevation you need.
The RMS error estimate for the orthometric height
includes an error estimate for the GEOID in addition to
the RMS value for the ellipsoid height.
US Survey Feet vs. International Feet,
Scale Factors
The state plane coordinates are listed at the bottom in
the right-hand column. They are in Meters. If you need
Feet, you can convert them, however be careful to
convert to International Feet or U.S. Survey Feet as
required by your State and application:
US Survey Feet
= Meters * (3937/1200)
International Feet
= Meters / 0.3048
The misapplication of Ft/M scale factor can result in a
30 foot coordinate blunder! If you request an ‘Extended
Format’ OPUS results, the state plane coordinates are
computed and returned at the bottom of the report.
If your survey is at a significant elevation (> 100 feet)
you may need to apply the Combined Factor (listed on
the OPUS report for both UTM and State Plane
Coordinates) to inversed distances to match optical
shots made at ground level.
Getting ready to use OPUS
OPUS is a great tool for grounding your survey. But
OPUS is part of a larger toolset. Before you begin a
project take a moment to think about the ‘Big Picture’:
A.
What are your GOALS?
a.
Required accuracy
b.
Horizontal and Vertical Datum; Geoid
model choice
c.
Survey style: OPUS-Static, OPUS-Rapid
Static, OPUS-Projects
d.
Consider FGDC Standards:
http://www.fgdc.gov/standards/projects/FGD
C-standards-projects/accuracy
B.
Are there passive marks available for control?
a.
Will CORS, passive or a combination
control the survey?
b.
Are local passive marks recoverable,
undisturbed, sufficient quality, stable
and GPS friendly?
c.
Where are the nearby NGS CORS (active)
marks? This will determine ‘Rapid Static’
or ‘Static’ availability.
C.
OPUS-Rapid Static Requirements
a.
Find the closest 9 CORS sites with
available observations
b.
A minimum of 3 CORS stations within
250 KM are required.
c.
Your site must be within 50 km of a
polygon created by the remaining
available CORS.
d.
If the eligible CORS count is low, check
the past reliability of recent observations
to insure that there is a high probability
of sufficient sites for OPUS-RS to
compute a solution. Use the CORS ‘Data
Availability’ to check for recent
observations:
D.
Mission Planning: Satellite Availability and
Network Planning
a.
How many receivers will you use for
simultaneous observations? If you are
using OPUS-Projects then More = More-
Better.
b.
Checkout online ‘Mission Planning’ tools
for U.S. satellite availability using
reasonable masks (>15 degrees) during
collection periods. If there are any
periods with fewer than 6 SV’s or PDOPS
higher than 3, plan on occupying points
longer.
E.
How long will you observe a site? Again:
F.
Are your sites GPS compatible? Are there
obstructions higher than 10 degrees?
G.
Field Checklist:
Maps, aerial photography, ingress/egress
plans
Receiver with memory available
Batteries fully charged plus battery-to-
receiver cables with 12V external battery
Compass for orienting receiver to North,
current declination adjusted
Fixed Height Tripods: Bubbles calibrated?
Height verified?
Tripods / Bipods / Tribrach / Tribrach
adaptors: tribrach calibrated?
Tools for adjusting bubbles (the correct
Allen wrenches)
