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GNSS Surveying

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GNSS Surveying

  • Global Navigation Satellite System
    • GPS (US), 32 in orbit
    • GLONASS (Russia), 24 in orbit
    • Galileo (EU), 30 in orbit
    • Beidou (China), 30 in orbit
  • WAAS (US) Wide Area Augmentation System developed by FAA to augment GPS
    • SBAS (US/Russia) Satellite Based Augmentation System (Inmarsat-4 F3, Galaxy 15, Anik F1R) Geostationary
    • GBAS (US) Ground Based Augmentation System- https://en.wikipedia.org/wiki/List_of_WAAS_reference_stations
  • Regardless of GNSS methodology you will get best accuracy with Post- Processing Kinematic (PPK)

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Important Specifications

  • Trimble Geo7x Centimeter Edition about $15k
  • Water Resistant/ Shock Resistant
  • Windows Mobile version 6.5
  • 4 GB internal memory and up to 32 GB SD card
  • GPS, GLONASS, QZSS, BeiDou, and Galileo
  • Centimeter level accuracy
  • Floodlight Technology
  • Laser Rangefinder

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Other GNSS options

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Ellipsoid, Geoid, and Datum

  • Ellipsoid is an oval with a major and minor axis
  • Spheroid is a 3D shape from a 2D ellipsoid (ie Clark 1866, GRS 1980)
  • Geoid is the surface of the earth’s gravity field (ie Geoid96, Geoid12B) which approximates mean sea level
  • Datum is built on a spheroid and incorporates local variation (ie NAD83, WGS84)

Geoid Height GEOID96

DATUM X-Coordinate Y-Coordinate

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NAD_1927 -122.466903686523 48.7440490722656

NAD_1983 -122.46818353793 48.7438798543649

WGS_1984 -122.46818353793 48.7438798534299

Note: Native format datum of GPS is WGS84. Newest is GEOID18, NAPGD2022 

(last ones were is GEOID12B, NAVD88).  Slide 1 (noaa.gov)

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How Specific You Need for Surveying Coordinates

  • Without Date, Time, Horizontal and Vertical Datum, Realization, and Epoch, points are worthless!
  • Good Example:
  • 14:04 GPST May 4, 2019, NAD83 Realization (2011) Epoch:2010.0000, 46.86105882, -113.86268819, 1004.630m NAVD88 Orthometric Height (GEOID18)
  • NAD83(2011)Epoch 2011, NAVD88, Geoid18

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Accuracy

  • Horizontal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 mm + 0.5 ppm RMS
  • Vertical. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 mm + 0.5 ppm RMS

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  • PPM- Parts per million, standardized measure of error in mm per 1000m

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  • So at 10 km from a base station (10 * 0.5) the error is 3 mm + 5 mm = 8 mm (about 1 cm) horizontal and 3.5mm + 5 mm = 8.5 mm vertical

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  • And at 100 km from a base station (100 * 0.5) the error is 3 mm + 50 mm = 53 mm (about 5 cm) horizontal and 53.5 mm vertical

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  • Basic rule of thumb: for every 10 km, you lose 1 cm of accuracy

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Base Stations

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  • Find your survey area and distance from base stations to find expected accuracy

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Survey Monuments

National Geodetic Survey Monuments- https://www.ngs.noaa.gov/NGSDataExplorer/

For each survey locate a NGS marker and take a point to check coordinate systems, elevation, etc. for QA/QC

Hint: If you have a choice, pick a “GPS and Vertical Control” marker for best accuracy.

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Ideal Base Station Configuration

Best Accuracy

Pretty Good Accuracy

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GNSS Accuracy with Survey Time

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Trimble GNSS

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Data collection

  • Terrasync is Trimble’s mobile data collection software

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  • Arcpad uses ellipsoid and is therefor useless for survey work.

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Real-time?

  • RTK- Real Time Kinematic- only really useful in MT near Butte, Billings, Missoula, and Bozeman
  • Integrated SBAS- Satellite Based Augmentation System. Improves accuracy from about 3-5m to 1-3m.
  • I have tried them all and it seems to be best to use Uncorrected GNSS and post-processed kinematic (PPK) to get cm level accuracy.

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Status Screen

  • Number of satellites

typically 15-30

  • PDOP- kind of a measure of accuracy best if < 3

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Collecting points

1) Create Data File

2) Select type of data and watch accuracy

3) Start collection

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Estimated accuracy

  • Tapping on the icon gives you estimated post-processed accuracy
  • You are getting the best you can if it says 2cm horizontal accuracy and 3cm vertical accuracy.
  • Once you see the 2cm/3cm accuracy start collecting data.

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iGage GPS

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iGage Collecting a Point

  • Use a field notebook, there is no user interface
  • Point ID, Description, Height of Antenna, Start Time, End Time
  • Point buttons to the north
  • Push and hold on button for 1 second until lights flash
  • After 30 seconds the blue light will flash once for every tracked satellite
  • Verify that the yellow Files LED flashes once every 5 seconds (or 1 second if configured) as the receiver logs data
  • Let it record for at least 16 minutes
  • Turn off by pushing and holding off button for 1 second.

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iGage Notes

  • You don’t need to use the record button, it happens automatically when turned on. After 30-90 seconds the yellow files starts to flash every 5 seconds, check the blue light to see how many satellites, you want >5.
  • Holding the record button for 2 second toggles on/off data recording
  • Tapping the mode button will cause either the serial or files button to flash
    • Serial green receiver is recording Files yellow receiver is NOT recording
  • There are four LED indicators on the receiver:
  • Power (Red) Power is ON. If flashing, the battery charge is very low.
  • SV’s (Satellite Count) (Blue) Blinks once for each tracked satellite, waits 5-seconds, repeats.
  • Serial (Green) Blinks when data is received by the serial port. Blinks when the ‘Record’ button is pressed if the receiver is storing to a static file.
  • Files (Yellow) Blinks each time data is stored to the static file default is each 5 seconds.

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GNSS Surveying Links

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Ground Control Points (GCP), Quality Control/Check Points (CP)

  • You can create models without GCP/CP points but accuracy will be something like 10 inches.
  • Ground control points are used to correct model to centimeter level accuracy.
  • Quality control points (check points) are used to objectively evaluate the model accuracy.
  • Rule of thumb, 5 control points per UAS flight, 5 quality control points per UAS flight, so 10 total per flight for high accuracy 3D models.
  • https://www.asprs.org/a/society/committees/standards/Positional_Accuracy_Standards.pdf

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Post-Processing Trimble

  • Pathfinder Office is Trimble’s software for post-processing
  • In general, you need to wait 24 hrs before post-processing

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Differential correction

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iGage Post-Process

  • Install download tool from iGGPS.com
  • Turn on receiver, wait 10 seconds, then plug into computer with usb
  • Start iGX download tool and select ‘Download from GPS’

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Submitting to OPUS

  • Highlight the occupation, set the point ID, description, and antenna height
  • Select submit to OPUS
  • When prompted select ok, then control-v to paste the file name and location, then press enter or click on open to set the zip filename
  • Manually enter the correct antenna ‘IGSIG4 NONE’
  • Click ‘Upload to Rapid Static’
  • What is OPUS? https://www.ngs.noaa.gov/OPUS/

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Downloading iGage Files

  • After you choose download it will display .HCN binary files
  • Enter your PID, description, antenna height, operator, and agency

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Exporting to Non-OPUS Programs

  • Files submitted to OPUS are decimated and striped of some info
  • Open configuration tab in iG4 occupation file
  • Choose show advanced settings= normal
  • An Export RINEX file will be shown in the occupations tab
  • When you click you get a full RINEX file with correct user, agency, antenna height and type in the header