Continuously Operating Reference Stations CORS

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Continuously Operating Reference Stations (CORS)

• Overview of Height Modernization Project
• Definition of CORS
• Different uses of CORS
• CORS Datum

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Alabama Height Modernization

• Funding from Congress for Alabama sponsored by
  Congressman Cramer
• Program managed by Department of Commerce NOAA
  - NGS
• Alabama Department of Revenue is grant recipient
• Surveying Tasks Delegated to Alabama Department
  of Transportation

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What is Height Modernization?
Wisconsin Height Modernization Project
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Height Modernization Activities assigned to ALDOT

• GPS Surveys using procedures from NOS NGS 58 59
  2 cm repeatable elevations from GPS.
• First and Second Order Leveling to support NOS
  NGS 58 59 Surveys and to restore and verify
  lines throughout the state.
• CORS Establish a state-wide CORS network to
  serve as the National Spatial Reference System
  (NSRS) for the state in the future and to provide
  data for monitoring crustal motion See
  Louisiana and NOS 50 4-D Positioning

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GPS

• GPS projects are performed using the NOS NGS 58
  59 procedures to obtain 2 cm heights.
• Primary Base Network A Order Horizontal and
  vertical marks observed on a 40 to 50 kilometer
  grid. Three 5 ½ hour sessions.
• Secondary Base Surveys in 9 areas of the state.
  Observations are complete and data is being
  processed. Final processing will be completed
  when leveling is completed. New and existing
  marks observed on a 12 to 15 kilometer grid.
  Emphasis on vertical component and 2 cm
  repeatability on GPS elevations.
• Local Base Surveys Basically the same
  procedures as the Secondary Base Surveys on a 4
  to 6 kilometer grid.
• Re-establish Azimuth mark for A Order project
  Back azimuth or mate was not observed and has
  resulted in azimuth errors on the ground.
  Preliminary planning is underway to re-establish
  all of the azimuth mates to the new A order marks.

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2003
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2006
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Goals

• Primary Base Network Horizontal and vertical
  marks established on a 40 to 50 kilometer grid.
  Attempt was made during reconnaissance to utilize
  as many benchmarks as possible. Leveling
  projects will place elevations on the remaining
  marks so that each A order mark has a first or
  second order elevation. These marks will be the
  foundation for the NOS NGS 58 59 surveys.
  Leveling began in May 2005 in Southeast Alabama
  (Dothan area).
• More good elevations on good GPS Marks better
  Geoid Model. Once leveling is complete to the
  Primary Marks, we will request that NGS define a
  new Geoid Model for Alabama.
• Better Geoid Model Better GPS elevations

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Leveling Objectives

• Put good NAVD 88 Heights on good Horizontal Marks
  observed with GPS
• Place good NAVD 88 Heights on CORS Antenna
  Reference Plane or ARP.
• Extend Leveling to areas with poor/non-existent
  benchmarks.

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Reconnaissance shows that up to 2/3 of Benchmarks
are destroyed or cannot be found!
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Leveling Projects to support Primary and
Secondary GPS Projects

• 44 miles Monroe, Clarke Escambia
• 20 miles Dallas, Lowndes
• 48 miles Marion, Franklin
• 10 miles Lawrence, Limestone
• 15 miles Talladega, Clay
• 34 miles Elmore, Montgomery
• 14 miles Macon
• 30 miles Coffee, Geneva, Henry
• 85 miles Remaining Primary Marks
• 340 miles total

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Leveling Projects to CORS stations

• Second Order Class I leveling to the Antenna
  Reference Plane (ARP) of the CORS
• Field work is complete for Huntsville, Tuscumbia,
  Birmingham, Tuscaloosa, Auburn, and Troy
• Preliminary planning underway for Alex City,
  Montgomery, Dothan, and Mobile.
• Establish NAVD 88 Orthometric Elevations at each
  CORS station.

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CORS

• Continuously Operating Reference Stations CORS
• Permanently mounted dual frequency GPS receivers
  connected to a recording device and operates 24
  hours a day, 7 days a week, 52 weeks a year with
  the raw data available via the internet and Real
  Time corrections available via wireless cell
  modem, radio signal, or both.

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• The Global Positioning System (GPS) is a set of
  satellites that enable you to determine your
  position anyplace on earth, 24-hours a day
  regardless of the weather
• The system consists of 24 operational satellites
  and four non-operational spare satellites
  orbiting 12,000 miles above the earth
• The satellites are arranged so that several
  satellites are always in view from any point on
  earth.
• The Global Positioning System (GPS) is based on
  the Department of Defense NAVigation Satellite
  Timing and Ranging (NAVSTAR) satellites.
• The service is free and is provided 24 hours a
  day world wide.
• GLObal NAvigation Satellite Systems (GLONASS) is
  provided by the Russian Federation
• European Galileo system is currently being
  developed and will add a third set of satellites.

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GPS Space Segment

• 24 Operational satellites (28 on orbit)
• Six orbital planes at an altitude of 12,600
  statute miles
• Satellites are spaced in orbit so that a minimum
  of five satellites will be in view to users
  worldwide
• Each satellite completes an orbit once every 12
  hours and will pass over head twice a day.

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Global Positioning SystemGPS
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Uncorrected GPS

• Hand held receivers
• Personal Digital Assistant (PDA) systems
• Lap Top based computer systems
• Uses code (L1) information
• Does not need base station or corrections
• Accuracies between 6 and 11 meters

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GPS Modernization Plan
Block III
Block IIR-M, IIF
Block IIA/IIR

• III IIF capabilities
• Improved civil signal (L1C)
• Increased accuracy (4.8-1.2m)
• Evaluating integrity improvements
• Navigation surety for military
• Increased A/J power (20 dB)

• IIR-M IIA/IIR capabilities
• 2nd civil signal (L2C)
• New military code
• Flex A/J power (7dB)
• IIF IIR-M capability plus
• 3rd civil signal (L5)

• IIA / IIR Basic GPS
• C/A civil signal (L1C/A)
• Std Service, 16-24m SEP
• Precise Service, 16m SEP
• L1 L2 P(Y) nav

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Basic Civil Positioning Before May 2000
100 m
GPS I, GPS II A, GPS II R

• C/A Code on L1
• Selective Availability On

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Basic Civil Positioning Now
6-11 m
GPS II R

• C/A Code on L1
• Selective Availability Off

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Basic Civil Positioning By 2009
3-5 m
GPS II R(M)

• C/A Code on L1
• Civil Code on L2

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Basic Civil Positioning By 2014
Better resistance to interference
1-3 m
GPS II F

• C/A Code on L1
• Civil Code on L2
• New Code on L5

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Basic Civil Positioning 2018
?
GPS III Full Operational Capability
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Differential or Static GPS Surveys

• Dual frequency GPS receiver tracks CA code,
  carrier and L1, L2
• Needs minimum of five satellites for
  initialization, then four
• Must have one or more base stations occupying
  known positions
• 1-2 centimeters IF proper procedures are followed.

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Why CORS?

• One Base Station for an area can serve many
  projects
• Reduced personnel and equipment costs
• Consistent coordinate datum
• CORS will eventually be National Spatial
  Reference System (NSRS)

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CORS Group

• Alabama Society of Professional Land Surveyors -
  ASPLS
• Geospatial Training and Application Center GTAC
• Troy University Geomatics Program
• GPS users in the First, Second, and Fifth
  Divisions

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Why would ALDOT maintain the CORS Network?

• Secure facilities throughout the state.
• Long term commitment to maintain sites.
• Each facility has generator back-up power in the
  event of outages
• Each facility has a T1 line for data transmission
• Each Division has IT personnel to assist in the
  maintenance of the stations.
• CORS network could be easily absorbed by ALDOT at
  a lower cost.

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Objectives for a CORS Network

• Short Term
• Improve coverage for Static GPS Work
• Leveling to establish NAVD 88 Orthometric Heights
  at the Antennae Reference Plane (ARP)
• Long Term
• State-wide coverage at 70 km spacing - Sufficient
  Density to allow network corrections
• Real-Time corrections made available to
  Geo-Spatial Community
• Improved GPS Elevations

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• Phase I
• Evaluate Hardware, Software, installation methods
• Install 4 stations in urban areas.
• Install 1 station in vicinity of tide gauge at
  Dauphin Island
• Provide real-time, single base-line solutions via
  internet
• Phase II
• Add 6 (GG) CORS stations (2 were installed in
  October)
• Develop partnerships with County Government to
  establish Co-op CORS
• Continue leveling efforts to establish NAVD 88
  Heights to the ARP
• Phase III - VI
• Continue to add 4 to 6 stations a year
• Develop partnerships with County Government to
  establish CO-OP CORS and build redundancy in the
  system.
• Implement network solutions when sufficient
  density is reached.

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NGS CORS Guidelines

• GOALS
• Improve quality of CORS data
• Focus attention on CORS meta-data requirements
  more detailed Site Logs and Photos

Selected Items Team established to evaluate
new sites more rigorously before
including them into the CORS network
Encourage more stable sites Avoid
multipath-prone sites Radomes not
recommended except Track all satellites
regardless of their health status Track
satellites to lower elevation angles Provide
suite of photos for each site
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Challenges of Implementing CORS

• Site selection
• - Must be a stable site with no obstructions
  above 5 degrees
• - Must be within 30 meters of adequate network
  and power connections
• - Long term survivability trees will grow and
  possibly obstruct antenna. Possible building
  additions/renovations must be considered.

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Grangeville Idaho
Michigan DOT Towers
Tower Design
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Challenges of Implementing CORS

• Monument Design
• - The ideal monument requires a large clear
  area with no possibility of future obstructions.
• - Easy to establish NAVD 88 Elevation at ARP
• - Roof-top mounts have proven to be very stable
  and get the antenna above most obstructions.
• - Can be difficult to establish NAVD 88
  Elevation to the roof.
• - The Louisiana network utilizes roof mounts.
  One site was lost during Hurricane Katrina

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Tuscaloosa AL50
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Birmingham AL30Roof mount at 3rd Division
Troy AL70Roof mount at 7th
Division Office
Tuscumbia AL20Roof mount at 2nd
Division Office
Montgomery AL60Roof mount at 6th
Division Office
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Not picturedHuntsville GTAC Roof MountAuburn
ALAU Roof Mount
Mobile AL90 - ProposedRoof mount at East
Tunnel Office
Dauphin Island ALDI Roof
mount at Sea Lab
Alexander City AL40Roof mount at
4th Division Office
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Choke Ring vs. Geodetic Antenna

• Choke ring has typically been thought of as a
  more robust antenna.
• Recent evaluation of the GPS data at several
  sites has shown that the Radome can cause errors
  in the position if not properly calibrated.
• New Geodetic-quality Antenna shows near choke
  ring quality Time will tell.

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Challenges of Implementing CORS

• Data Communication
• Reliable data connection to provide raw RINEX
  files from the receiver to the NGS FTP site for
  static processing as a minimum.
• Ideally we need to provide an adequate data
  connection to send raw data from antenna to main
  server in Montgomery, calculate corrections, and
  sent back out to the internet fast enough to be
  used for real time corrections in the field.

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Data transmission methods back to the Rover

• Pac-Crest Radio
• 35 watt high wattage, high heat.
• Requires FCC License.
• Typically a four mile radius based on terrain.
• Higher cost per unit up front, but lower long
  term costs.
• Possibly get the antenna higher to provide larger
  coverage area?
• Cell modem
• Low front end cost for equipment
• Monthly cell bills for unlimited minutes add up.
• Poor and spotty coverage in rural areas.
• Wait for coverage to catch up?
• 900 MHZ
• Lower wattage less heat
• Requires line of sight to repeaters and rovers
• Other?
• High Frequency band-width with high wattage
  transmitter (and high cost)? More difficult to
  obtain FCC licensing.
• WI-MAX?

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User Issues

• Each user will have to evaluate the areas where
  the majority of their work will be performed to
  determine which cell provider gives the best
  coverage.
• Each user will have to evaluate existing
  equipment to determine upgrades necessary to
  communicate with CORS Server.
• Vendors are eager to work with surveyors to
  upgrade existing equipment or sell new equipment.

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Louisiana

• System of benchmarks has been deemed unreliable
• Transfer NAVD 88 orthometric heights to CORS ARP
• Allow CORS network to serve as the vertical
  control for areas of subsidence
• Extent of subsidence into Alabama is unknown at
  this time. Project is being planned to determine
  the extent of the subsidence along the Gulf Coast.

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Tidal Gauges Dauphin Island

• NOAA Temporary Gauge

1968 NOAA Gauge
USACE Gauge
- Tide Gauges are tied to the Tidal Benchmarks.
Each site has several marks in close proximity to
the gauge. - Establish First or Second Order NAVD
88 Height at the Tidal Benchmark to provide an
accurate tie to Tidal Datum. - Establish NAVD 88
Height at the CORS ARP
Tidal Benchmark
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Dauphin Island CORS Site

• Finding a suitable site
• Limited access to structures with suitable
  horizon and stability.
• Providing adequate power and data communication.
• Design and build a mount to reasonably withstand
  weather and corrosion.
• Close proximity to Tidal Benchmarks and First
  Order NAVD 88 Benchmarks.

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Dauphin Island CORS Site Data and Power
considerations

• Computer Network at Sea Lab is not reliable in
  good weather.
• - Look at the possibility of using cell modem to
  transmit the data back to the server?
• - perform on-site testing with the equipment to
  ensure there is sufficient cell coverage and data
  transmission speed.
• Power is susceptible to bad weather.
• - Provide battery back up to cover power
  outages.
• - Prepare to install solar panels and batteries
  at this or any site in the event of catastrophic
  power loss.

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Existing Level Lines
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• Time series at fort Morgan CORS prior to 2005
  HARN project

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Latest Velocity rates from Fort Morgan CORS after
2005 HARN Project
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Southeast CORS Network May 2005
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• Alabama CORS October, 2006

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Benefits of a CORS Network

• Data files archived for post-processing of static
  GPS work Survey and Engineering Control Work
• Real-Time corrections supplied to collectors of
  geo-spatial data GIS systems
• Precision Agriculture machine guide
• Construction machine guide
• Emergency response
• Navigation
• Air-Borne GPS
• Monitor possible crustal motion throughout the
  State
• Eliminates need for Base Station at remote
  locations
• Reduced equipment costs
• Consistent positions
• Future vertical control network
• Satisfy most positioning needs that require
  centimeter level accuracy!

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Aerial Mapping, photography, and Digital Terrain
Models

• Good horizontal and vertical control is REQUIRED
  to obtain ACCURATE Digital Ortho Photographs and
  Digital Terrain Models

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Basis or Foundation for an ACCURATE GIS
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LAND SURVEYING
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SAFETY / FLOODING

• Coastal Erosion

• Flood Stage Predictions
• and Measurements

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• Remote Sensed Imagery
• Instrumented Farm Implements
• Crop yield monitoring equipment
• Light Bar and Auto-Steer guidance systems
• Results in increased crop yields, reduced
  production costs, and reduced runoff of
  fertilizer and topsoil

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HIGHWAYS / TRANSPORTATION

• Better Decision making for Route Location,
  Roadway Maintenance and Improvements

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Uses of CORS Data

• Static Post Processed using OPUS or Vendors
  software
• Real-Time corrections transmitted directly to the
  rover in the field.

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Static GPS

• Raw data is collected at the receiver for two
  hours or more observing two sessions on each mark
• Data from CORS sites are downloaded and
  post-processed with the vendors software

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Static GPS using OPUS

• Raw data is collected at the receiver for no less
  than one hour OPUS still prefers two hours of
  data.
• Raw data is sent to On-line Positioning User
  Service OPUS.
• Solution is emailed back to the user usually in
  less than five minutes.
• Option has recently been added for the user to
  select the base stations to use for the
  processing.
• NGS is working on routines that will process
  fast-static sessions and single frequency
  sessions.
• There are also plans for an OPUS database that
  would contain all of the OPUS solutions. This
  data would be retrievable in much the same way a
  conventional data sheet is retrieved.
• NGS is working towards supporting all available
  satellite signals.

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OPUS On line Positional User Service
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How Does OPUS Compute Position?
3 separate baselines computed 3 separate
positions averaged
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Looking Forward

• NGS is Developing OPUS-RS (rapid static) that
  will enable users to obtain positional
  coordinates with cm-level accuracy using only 15
  minutes of GPS carrier phase data.
• NGS is developing OPUS-DB (OPUS DataBase) which
  will allow users to submit GPS data files,
  observation log, photos, and description. These
  marks will be made available much the same way a
  datasheet is obtained via internet.
• NGS is exploring the use of NTRIP to stream GPS
  data from selected CORS via the Internet.
• Equipment manufacturers have released new
  receivers capable of receiving GLONASS, Galileo,
  L2C, and L5.

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CORS 96 vs. NAD 83/92

• Comparisons of OPUS positions against published
  positions led ALDOT to believe that there is up
  to a 5 cm shift between the CORS 96 and 83/92.
• Prior to the HARN observation, NGS stated that
  the 07 adjustment would adjust the 83/92 Datum,
  which are the positions currently in the
  database, to the CORS Datum.
• The processing for the HARN Observation indicate
  that all of the existing A and B order marks
  observed fell within the Network Accuracy limits
  of 5 cm, therefore, no adjustments were made to
  the existing A and B order positions.
• The CORS positions and the existing A and B order
  positions were held as fixed for the HARN project.

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2003
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2006
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Problems

• New A Order positions have been posted to the NGS
  database.
• - 32-6 in Greene County shifted by 2 decimeters
  (0.66 feet).
• - The azimuth to back-sight mate changed by
  0-00-50!
• - 2007 Adjustment should correct this?
• - Use the back sight mates to the new A order
  stations with caution!

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Disclaimers

• ALDOT does not guarantee ANY position computed
  from the CORS sites! It is the surveyors
  responsibility to ensure all field and office
  procedures have been followed to ensure the data
  is correct.
• NGS will establish the position of the CORS and
  it will be checked daily.
• Tuscumbia, Tuscaloosa, Troy, and Birmingham are
  running now with temporary positions established
  by the vendor. NGS will provide positions after
  inclusion into the National CORS program.
• It is the surveyors responsibility to verify any
  position computed or calculated with any CORS
  station.
• Repeat Observations the best way to verify a
  position is to repeat the session under a
  different satellite constellation a minimum of
  four hours separation between the end of the
  first session and the beginning of the repeat
  session.
• To obtain 2 cm vertical, see NOS NGS 58 59.

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Disclaimers

• ALDOTs first priority is the collection of data
  to meet the requirements for the National CORS
  program.
• ALDOT will attempt to maintain a continuous,
  un-interrupted flow of data, BUT we cannot
  guarantee it.
• A web site will be established that will report
  the condition of the sites and a schedule of
  proposed maintenance or down time to the site. A
  link will be provided on the ASPLS web page to
  this site as soon as its available.

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CORS Sites

• Existing Public Sites
• GTAC Huntsville Space and Rocket Center
• ALAU City of Auburn
• ALDOT Sites to be posted as National CORS by
  November 06.
• AL20 Tuscumbia Second Division Office.
• AL30 Birmingham Third Division Office.
• AL50 Tuscaloosa Fifth Division Office.
• AL70 Troy Seventh Division Office.
• ALDOT Sites under construction awaiting
  approval from NGS.
• ALDI Dauphin Island Operational by November,
  2006.
• AL60 Montgomery Operational on October 6, 2006.
• AL40 Alexander City Operational on October 12,
  2006.
• Proposed Sites
• AL90 Mobile East Tunnel Office Begin install on
  October 23, 2006.
• AL12 Gadsden Begin install on October 30, 2006.
• AL52 Chilton County no install date determined
  at this time.

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• We are requesting that each user register with
  us to obtain IP address and port numbers so that
  we can build a contact list of active users.
• Notification of proposed work or downtime at the
  sites.
• Notification of any changes to the system that
  may affect use.
• Notification of new sites.
• Attempt to get an idea about the number of users
  for future justification of funds.
• Establish contact lists for forums, educational
  opportunities, or input from the surveying
  community.

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Requested Information

• Send an email to russellj_at_dot.state.al.us. In
  the email message, please provide the following.
• Name, address, phone number
• Type equipment (GPS, GLONASS)
• How will you use the system Static? RTK?
• Number of users
• List of areas you typically work in.

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Questions?
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Project Control on Location Surveys
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Location Section

• GPS used to establish horizontal control set at
  one to two mile interval along project corridor.
• Conventional Traverse between GPS pairs to
  establish control at approximately 1000
  intervals.
• Conventional leveling from First or Second Order
  benchmarks through GPS and traverse marks to
  bring vertical control into the project.

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Benefits

• Solid foundation of horizontal and vertical marks
  to support the project from corridor study
  through construction.
• Ability to utilize mapping, photography, and GIS
  data from any source as long as it is in a known
  datum.
• Eliminated physical ties between projects.
• Eliminate Coordinate Equations
• If project control is destroyed in any phase of
  work it can easily be restored. Control points
  are never Lost.

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Establishing Project Control

• Search database for horizontal and vertical marks
  in the area.

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• Datasheet shows the mark name, lat/long,
  horizontal/vertical order designations

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• Latest NGS Datasheets show coordinates in meters
  in the appropriate zone
• Recently began showing Combined Scale Factors
• This area has a ratio of 1 part in 20,000 grid
  vs. ground distances.

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• Datasheet shows the type monument, stamping,
  satellite visibility, and recovery history.

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• Datasheet shows recovery history from initial
  monumentation to the latest recovery notes.

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Static GPS Observations

• 18 Rebar w/ cap stamped with point number and
  year set
• Set in an area with a good horizon for satellite
  visibility
• Set 4 to 6 underground to avoid damage by
  mowers and other hazards

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Numbering Schemes

• 1 County number with letter designation for
  marks established with GPS.
• 2 Project specific point number with year for
  marks established with total station.

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Description Writing

• New mark is worthless unless properly described
• Odometer log with ties to mile posts
• Measurements to obvious objects around each point

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GPS Observations

• Leica 1200 Survey Grade Receivers
• 1 hour 15 min sessions
• Observation logs
• Each new mark is occupied twice
• Side ties are occupied three times
• Enough over-kill to avoid any return trips or
  errors.

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Leveling

• Once marks are set Crew splits up and three men
  begin leveling while the other make the
  observations.
• Every new mark is leveled through
• Leica DNA 03 First Order level
• Third Order Elevations established on each mark
• Begin and end on Second Order, Class II Benchmark
  or better

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Traverse

• After GPS observations are complete, remaining
  crew members begin conventional traverse through
  project.
• Marks are set at 1200 intervals
• Leica TCA 2003 0.5 electronic theodolite with
  a 1mm 1ppm distance meter
• Servo driven with Automatic Target Recognition
• Angle is turned five times
• Distance is taken five times each direction for a
  total of 10 distances per leg

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Computation

• Azimuths from GPS positions are used for
  beginning and ending azimuths
• Software means horizontal and vertical angles
• Distances are meaned manually (software will not
  currently do this)
• Traverse is computed using an old Geodimeter
  program which computes azimuth closure and closes
  the angles before giving error of closure
• Calculates Combined Scale Factor for each leg of
  the traverse and reduces the horizontal ground
  distance to a grid distance.

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Fast Static GPS Observations

• Not part of a control survey
• Post-processed and computed in the same way
  static observations are processed
• Used for DTM Topo Survey
• Two base stations on known marks collecting data
• Rovers collect 10 to 15 minutes of data based on
  satellite geometry

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Real-Time GPS Observations

• Not part of a control survey
• Provides position and elevation as soon as the
  point is recorded
• Used for DTM or Topo operations
• Generates ASCII file
• For best solution, a Site Calibration should be
  performed ?

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Real-Time Data Collection

• Rover collects data independently of base
• CORS will replace the base and the cell modem
  will replace the 35 watt radio link.

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Site Calibration

• Base Station is set on Project Control with good
  horizon
• Using the rover, collect 3 minutes of data on
  each project control with suitable horizon
• Coordinates for each point are entered into
  collector
• Collect data on points within and around the
  project

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Site Calibration

• After data is collected, software will produce a
  list of the points with the RMSE for each point
• Points with larger RMSE may be removed
• Be careful not to remove the endpoints and that
  you are not relying on midpoints within the
  project
• Software will calculate a coordinate system for
  the project, name it, and store the system
• Coordinate system can be transferred between
  receivers

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Site Calibration

• Time consuming work at the beginning of the
  project to establish localization to the project
• Most accurate and reliable method to use
  real-time
• Performed once on each project
• Calibration files are stored and saved for future
  work

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Real Time GPS Observations

• Horizontal and Vertical Accuracy limits can be
  pre-set
• Normally use 0.05 ft for hard surfaces and 0.1 ft
  for ground shots
• Each rover can work independent of everything
  except the radio or cell link
• Does not require line of sight or communication
  with instrument man
• Does not go under tree canopy very well.

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P4
P1
P2
P5
P3
P6
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P4
P1
P2
P5
B9999-1
B9999-2
P3
P6
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Issue to be Resolved

• Is Accuracy of single base-line RTK solution
  comparable to Site Calibration?
• Users must evaluate the data and results.

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Questions?