Surveying with a GPS
How to configure a GPS unit to ensure the
highest accuracy
Trimble and Terrasync
• Trimble uses proprietary software to
configure and collect spatial data
– Terrasync
• Pathfinder Office
– Differentially correct data
– Create data dictionaries
– Export data for use in ArcMap
Configuration for Survey
• For the greatest
accuracy the unit
must be
configured
properly.
Logging Setting
• If using an
external antenna
make sure height
is set.
• Point/Vertex
Auto-pause
count
– Set to 10
GPS Settings
• Uncheck box next
to Productivity –
Precision bar
• DOP type should
be horizontal
Accuracy and your GPS
• Position Dilution of Precision (PDOP)
– Value that indicates when satellite
geometry can provide the most accurate
results
– The greater the spread the better
– A low PDOP measurement is desirable
– PDOP mask is set at 6
Accuracy and your GPS
• Elevation Mask
– Restricts the receiver to using only SV’s
above a certain elevation in the sky
– Default set to 15 degrees
– How will lowering this mask impact your
accuracy?
• Signal-to-noise ratio (SNR)
– Measure of the information content of the
signal relative to the signal noise
• The higher the value the better
Real-time Settings
• Integrated SBAS
is the WAAS
system
Correction method
• Real-Time Differential
– A base station continuously broadcasts
corrections via a radio signal
– Often referred to as RTCM (Radio Technical
Commission for Maritime Services)
– Post processing a real-time differential
data set can still improve accuracy
Carrier Phase
• The most accurate method for data
collection on our GPS units
• Requires more rigorous data collection
techniques
– Accurate antenna height
– 5 SV and a clear view of the sky
– Within 50 km of a base station
Coordinate System
• System
– US State Plane 1983
• Zone
– North Carolina 3200
• Datum
– NAD 1983 (CONUS)
• Altitude Units
– Survey feet
• Geoid Model
– Other
• Geoid
– DMA 10X10 (Global)
• Altitude Reference • Coordinate Units
– Mean Sea Level
(MSL)
– US Survey Feet
• Display USNG
– Off
Units
• This window is
set to your
preferences.
• It will not affect
the accuracy of
data
– Use the units you
prefer
Create an new file
• Assign a unique
file name
• Can also open an
existing file or the
file manager