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The Global Positioning System IIField Experiments

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Mexico DGPS Field Campaign

Cenotes in Tamaulipas, MX, near Aldama

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Are Cenote Water Levels Related?

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DGPS Static Survey of Cenote Water Levels

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Determining Orthometric Heights

• Ortho. Height = H.A.E. – Geoid Height

Earth Surface

Ellipsoid Geoid

Height

Height above MSL

(Orthometric height)H.A.E.

Geoid height

= H.A.E.

Geoid

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Determining Orthometric Heights

• Ortho. Height = (H.A.E. – Geoid Height)

Need:

1) Ellipsoid model – GRS80 – NAVD88• reference stations: HARN (+ 2 cm), CORS (+ ~2 cm)

2) Geoid model – GEOID99 ( + 5 cm for US)

Procedure: Base receiver at reference station, rover at point of interest

a) measure HAE, apply DGS corrections

b) subtract local Geoid Height

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Sources of Error

• Geoid error – model less well constrained in areas of few gravity measurement

• NAVD88 error – benchmark stability, measurement errors

• GPS errors – need precise ephemeri, tropospheric delay model, equipment (antennae should be same for base and rover)

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Static Carrier-phase solutions obtained by:

• Commercial post-processing software

• e.g. Trimble Pathfinder office

• Web-based Precise Point Position Services

• Automated Gipsy – JPL/NASA - global

• SCOUT – Scripps, UCSB - global

• OPUS – NGS (US and territories)

• All services require files in RINEX format

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Results

• Horizontal accuracies of <1 cm

• Vertical accuracies of 2-5 cm for 4 hrs of data

GPS Applications Today

• Surveying – Tectonics, Cadastre, Geodesy

• Map Making – georeferencing, field studies

• Navigation – vehicles, missiles, robots, etc.

• Tracking – people, vehicles, pets

• “Geotagging” – apply coordinates to digital data (photos, etc.)

• Clock Synchronization (+ 10 ns)

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GPS and Geologic Mapping

Two techniques:

• GPS receiver and separate, gridded paper maps

• “Mapping-grade” receiver with mapping software and interactive touch screen

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Low-Tech Mapping

Gridded maps/photos, pencil, GPS receiver

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Meters

Contour Interval = 20 feet

UTM NAD83 13NSpence 7.5 ' Quad. Õ16 degrees

True N is ~1 degreesEast of grid N.

1:12,002

MN

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High-Tech Mapping Tools

Field Hardware

Mobile Devices

Outdoor tablet computers – Windows

Field GIS/GPS Software

ESRI Software

ArcPad

ArcMap

Collector App

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GPS-equipped Windows Mobile Devices

• Small, low res., color screens

• Limited CPU

• Resistive stylus pen

• Up to 12 hrs. battery life

• Light, portable

• Variably rugged

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Outdoor Tablet Computers

• Large, outdoor-viewable,

high res. color screens

• Inductive stylus pen

• 3-4 hr. battery life

• Heavy, rugged

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GIS-GPS Software

ArcPad

Uses standard data formats – shapefile, tif, SID, etc.

Good GPS support

Supports custom forms

Well-integrated with ArcGIS products

Easy to use

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Tablet Running ArcPad

GPS Location

pCcm

pCdmQal

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ArcGIS - ArcMap Software

Desktop software in the field

Powerful Drawing tools – best feature

Edit Geodatabases in the field

Minimal GPS capture tools

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Tablet Running ArcMap

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High-Tech Mapping Tools

Field Hardware

Mobile Phones

Ipad or other tablet

Field GIS/GPS Apps.

Apple OS, Android OS Collector App

Survey123

FieldMove

iGIS

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High-Tech Mapping Tools

Mobile Apps PDF Maps

iGIS

Collector

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High-Tech Mapping Tools

App DistinctionsWill maps work offline?

Will GPS work offline?

Does it drop pins, geotag photos, notes etc.? E.g.

PDFMaps and many others free Apps

Does it allow capture of lines and polygons too? E.g.

iGIS, FieldMove, Collector and a few others; free to

$$

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Assisted GPS (A-GPS)

Mobile Devices with GPS and WiFi or Cellular Service, e.g. LBS (location-based services)-capable phone

1. GPS Almanac provided from Server; TTFFfaster – position found by phone (Mobile Station Assisted: “MSA GPS”)

2. GPS data sent to server, position sent back (Mobile Station Based: “MSB GPS”)

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Receiver attributes

• # of Channels

• One channel required for each frequency (L1, +/- L2)

• 8 minimum (4 SVs); 12 or more desirable

• Antenna

• Remote, fixed

• Power source

• Internal, external

• Data Storage

• Way-points vs. data logging

• Positions vs. raw data

• Data upload & download

• Data dictionary upload for storing positions by attributes (pt., line, area)

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Receiver attributes

• DGPS capable

• Beacon antenna for real-time DGPS

• Download and post-process

• WAAS capable

• Ionosphere Correction or model

• Dual channel vs. single channel receiver

• Troposphere model?

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GPS Equipment

Hand-helds $100-$450 – navigation instruments

Garmin

Magellan

GPS for PDAs

• Way Points collection

• Manual entry into GIS,

no attribute info. stored

• “Differential ready” but

no post-processing

For survey apps.:

+/- ~15 meters

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GPS Equipment

Sub Meter (x, y) Accuracy, Hand-helds: $1000-$5000

Trimble

Ashtech

Topcon

Sokkia

others

Datalogging with “data dictionary” to upload/store attribute info.

GPS signal data collection for later post-processing

Custom RTK and beacon antennae feasible

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GPS Equipment

Geodetic-quality Instruments

• Trimble

• Ashtech

• Sokkia

• Others

Cm – mm in x and y; 2 cm in z

Stationary Antenna

Large memory for continuous data collection

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Recent Developments

• Hand-held equipment – Field GIS

• WAAS, LAAS

• Other Global Navigation Satellite Systems (GNSS)

• European Union Galileo System (2014) and EGNOS SBAS

• Russia - GLONASS (23 SVs) + SBAS services

• Chinese BeiDou Navigational System (BDS); 14 SVs, 35 by 2020; +SBAS services)

• Regional Satellite Systems

• Gagan – India (2012)

• Japan –MSAS (QZSS?)

SBAS Service Areas

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SBAS = Satellite Based Augmentation SystemsSource: Wikipedia, 2015