GNSS DERIVED HEIGHTS- PART 2 NOS/NGS - 59 NGS WEBINAR – OCTOBER 7, 2009 Bill Henning Senior Geodesist, PLS. 301-713-3196 x 111, william.henning@noaa.gov

DAY 1 GNSS DERIVED HEIGHTS DAY 2 NOS/NGS-59- Baltimore County and Fairfax County REAL TIME HT ISSUES DAY 1 HT MOD GNSS HT METHODS NOS/NGS-58

http://www.ngs.noaa.gov/ “NGS 58” “NGS 59”

GEOID03 is a refined model of the geoid in the United States, which supersedes the previous models GEOID90, GEOID93, GEOID96, and GEOID99. For the conterminous United States (CONUS), GEOID03 heights range from a low of –50.97 meters (magenta) in the Atlantic Ocean to a high of 2.23 meters (red) in the Labrador Strait. However, these geoid heights are only reliable within CONUS due to the limited extents of the data used to compute it.

ELLIPSOID, GEOID & ORTHO HEIGHTS “h = H + N” Earth’s Surface P Ellipsoid Plumb Line h Q N Mean Sea Level “Geoid” Relationship of the mathematical ellipsoid surface with the previously developed diagram of level surfaces and orthometric heights. There is no relationship between the ellipsoid and the gravity field. It has nothing to do with level surfaces and its surface cuts through all level surfaces; not parallel. GPS heights are not related with the geoid - no relation to gravity field; required model to obtain differences between the geoid and ellipsoid to determine orthometric height. Ellipsoid is above the geoid in the U.S. - why its drawn this way; geoid always negative. Geoid height (separation, undulation) - difference between the geoid and ellipsoid at any given point on the earth’s surface. H = orthometric height, h = ellipsoid height, and N = geoid height. h=H+N - accurate to mm as long as all the components are known. PO Ocean h (Ellipsoid Height) = Distance along ellipsoid normal (Q to P) N (Geoid Height) = Distance along ellipsoid normal (Q to PO) H (Orthometric Height) = Distance along plumb line (PO to P)

Expected Height Accuracies GPS-Derived Ellipsoid Heights 2 centimeters (following NOS NGS-58 Guidelines) Geoid Heights (GEOID03) Relative differences typically less than 1 cm in 10 km 2.4 cm RMS about the mean nationally 0.5 cm error in 10 Km Leveling-Derived Heights Less than 1 cm in 10 km for third-order leveling Following developed guidelines, NOS NGS-58, produce ellipsoid heights to 2 cm. Latest geoid model produces relative differences typically less than 1 cm in 10 km. Inherent error sources in running levels produces 1 cm uncertainty in 10 km for Third Order leveling.

www.ngs.noaa.gov Available “On-Line” at the NGS Web Site: SEARCH: “NGS 58” Guidelines - will eventually become routine; not so much explanation of why its done but provides background information. Repeat baselines, station spacing, fixed height antenna setups, identify and control all error sources, tie local networks together, etc.. NOS NGS-58 GPS-Derived Ellipsoid Heights Guidelines will lay the foundation for GPS-Derived Orthometric Heights Guidelines. Produce 2 cm ellipsoid heights to be able to obtain 2 cm orthometric heights.

What has been accomplished using NGS 58? Control recovery, campaign planning/logistics Obs schedulules, equipment checks Observations: Repeat base lines Different days Different times of day (“15 hours, 27 hours”) (Detect, remove, reduce effects due to multipath and using the same satellite geometry) Recommendations - no low RMS; short data sets using a small piece of satellite visibility can cause high RMS; delete suspect satellites and reprocess - can’t fix integers - something’s wrong. Single frequency receivers will work - fix integers - short baselines - longer data sets. Different days - schedule 2nd observation on different day if reasonable. It’s just a matter of scheduling.

FAIRFAX COUNTY HEIGHT MODERNIZATION PROJECT 2004-2006

Vector Processing Accomplished Elevation Mask - 15 degrees Ephemeris - Precise (typ. 14 days latency) Tropospheric Correction Model Iono Corrections - All baselines longer than 5 km. Fix Integers Baselines less than 5 km: L1 fixed solution Baselines greater than 5 km: Iono free (L3) solution Baselines must have RMS values ≤ 1.5 cm Baselines must have difference in “up” ellipsoid height ≤ 2.0 cm 10 degree mask for data collection allows the receiver to “cleanly” lock on to low elevation satellites providing strong, continuous satellite lock when processing at 15 degrees. Precise orbits - maybe not much different compared to broadcast orbits but could be great - IGS orbits – ITRF97 positions, ±5 cm; combination of orbits from various sources - NGS is one supplier typically with a 13 day delay; use of “rapid orbits” maybe ok; available 1 day delay for special purposes. Ionospheric effects - can produce major height problems; eliminate, reduce, identify errors; LI/L2 differences - Iono free solution, combined L3 - removes effects caused by ionosphere. Most other errors created by ionospheric conditions are taken out in processing software. Ionospheric models work about the same above 15E; below 15 E is where the models differ. Standard tropo models; available in software. Wet troposphere - difficult to model out; how wet is wet? Nice weather; good results - bad weather; not so good results; 2-5 cm uncertainty; also affects multipath. Dry troposphere - can be modeled fairly well; required for long lines, different conditions, large height changes over short baselines. Can change heights by 3 to 7 cm through modeling. Recommend low RMS values, i.e., < 1.5 cm; short data sets using a small piece of satellite visibility can cause high RMS; delete suspect satellites and reprocess – if you can’t fix integers - something’s wrong.

Repeat Vector Analysis After Re-Processing From To Session dh Diff Dist RMS Solution Station Station Meters cm Meters Type BM20 04KU 076G 46.009 3628 0.015 L1 fixed double 077G 46.004 0.5 0.017 L1 fixed double 078R* 46.007 0.2 0.015 L1 fixed double ZINC PT14 078A 15.397 3173 0.006 L1 fixed double 077A 15.400 0.3 0.006 L1 fixed double 076A 15.408 1.1 0.006 L1 fixed double TIDE 04KU 078G 43.680 3133 0.022 L1 fixed double 077R* 43.654 2.6 0.024 L1 fixed double 076R* 43.658 2.2 0.020 L1 fixed double PT14 TIDE 077A -55.031 3765 0.022 L1 fixed double 078R* -55.027 0.4 0.023 L1 fixed double 076R* -55.019 1.2 0.018 L1 fixed double 04KU 5144 078G 28.939 7250 0.014 Iono free fixed 077G 28.947 -0.8 0.014 Iono free fixed 076G 28.940 -0.1 0.020 Iono free fixed 5144 ZINC 078A -33.045 6167 0.011 Iono free fixed 077A -33.051 -0.6 0.009 Iono free fixed 076A -33.063 -1.8 0.013 Iono free fixed Differences in GPS-derived height (dU) values between repeat baselines. BR13 to BR14 – float solution; RMS value is low but unable to fix integers. Try to reprocess or re-observe this baseline. Note that the second and third baseline comparison is very close. BR14 to GR15 – high RMS on all three baselines; obstructions, multipath? BR19 to CL20 – high RMS on all three baselines. Note the second and third baselines are much closer together. Reprocess all baselines that have repeat differences greater than 2 cm. *NOTE - Reprocessed vectors which had differences greater than 2 cm.

Adjustment Procedures to Obtain GPS-Derived NAVD’88 Orthometric Heights

Least Squares Adjustments The adjustment minimizes the effects of random errors A least squares adjustment computes a single network solution, even with redundant vectors Least squares will highlight blunders and large errors It will provide estimates on the precision of the coordinates for the stations

GPS ELLIPSOID HEIGHT HIERARCHY HARN/Control Stations (75 km) Primary Base (40 km) Secondary Base (15 km) Local Network Stations (7 to 10 km) Appendix B. Guidelines guarantee proper ties and provide acceptable relative accuracies. HARN not better than 5 cm vertical accuracy absolute. CORS getting down to 3 cm vertical accuracy absolute. Basic concepts - nutshell overview - if errors are not large they’re probably not visible; requires redundancy and a network. 5 cm accuracy - ties to the net at least to 5 cm (network) and internally to 2 cm (local). Layering of control to produce local network stations. Shorter baseline lengths, shorter observation times.

Project Adjustment Following Guidelines CORS HARN NAVD’88 BM New Station 121°40’W 122°35’W 37°50’N 38°20’N LATITUDE LONGITUDE Spacing Station Primary Base Station 8.2km

Adjustment of Primary Network Stations From Control Horizontal Adjustment (Latitude, Longitude, Ellipsoid Heights) Minimum Constrained [One fixed station] Fix latitude, longitude and ellipsoid height at one station Resolve all blunders and large residuals Determine which Control and known Primary Base Station coordinates should be fixed Constrained [All suitable stations fixed] Fix latitude, longitude, and ellipsoid heights at Control and known Primary Base Stations Make sure the constraints did not distort the project NOTE - Geoid model NOT applied at this time

FAIRFAX COUNTY CONTROL & PRIMARY STATIONS

Adjustment of Primary Base Stations CORS HARN NAVD’88 BM New Station D191 10CC 19.0km Primary Base Station 28.7km 25.7km LATITUDE 38.3km 31.6km 38.7km 25.8km LAKE 29.6km MART MOLA 37°50’N 122°35’W LONGITUDE 121°40’W Constrained Horizontally CORS, Control Points (known Primary Control) horizontal latitude, longitude, and ellipsoid heights No NAVD88 orthometric heights constrained at this time

Adjustment of Local Network Stations Horizontal Adjustment (Latitude, Longitude, Ellipsoid Heights) Minimum Constrained [One fixed station] Fix latitude, longitude and ellipsoid height at one station Resolve all blunders and large residuals Evaluate coordinates at Control and Primary Base Station should not be greatly affected by Local Station baselines Constrained [All suitable stations fixed] Fix latitude, longitude, and ellipsoid heights at Control and Primary Base Stations Make sure the constraints did not distort the project NOTE - Geoid model NOT applied at this time

Adjustment of Local Network Stations CORS HARN NAVD’88 BM New Station Spacing Station 38°16’N Primary Base Station LATITUDE 8.2km 37°55’N 122°20’W LONGITUDE 121°40’W Constrained Horizontally Existing and newly derived Primary Control horizontal latitude, longitude, and ellipsoid heights No NAVD88 orthometric heights constrained at this time

Combined Network Horizontal Adjustment Perform combined adjustment Control and Primary Base network along with local network Latitude, longitude, and ellipsoid height Use GEOID model to obtain geoid heights Make sure combined adjustment did not distort the project

Combined Horizontal Adjustment CORS HARN NAVD’88 BM New Station 121°40’W 122°35’W 37°50’N 38°20’N LATITUDE LONGITUDE Spacing Station Primary Base Station 8.2km Constrained Horizontally CORS, Control Points and existing and new Primary Control horizontal latitude, longitude, and ellipsoid heights No NAVD88 orthometric heights constrained at this time

FAIRFAX HORIZONTAL

BASELINE ADJUSTMENT SUMMARY

Repeat Vector Analysis From To Session dh Diff Dist RMS Solution Station Station Meters cm Meters Type BM20 04KU 078G 45.974* 3628 0.016 L1 float double 077G 46.004 -3.0 0.017 L1 fixed double 076G 46.009 -3.5 0.015 L1 fixed double ZINC PT14 078A 15.397 3173 0.006 L1 fixed double 077A 15.400 0.3 0.006 L1 fixed double 076A 15.408 1.1 0.006 L1 fixed double TIDE 04KU 078G 43.680 3133 0.022 L1 fixed double 077G 43.654* 2.6 0.024 L1 fixed double 076G 43.607* 7.3 0.020 L1 fixed double PT14 TIDE 078A -54.703* 3765 0.047 L1 fixed double 077A -55.031 -32.8 0.022 L1 fixed double 076A -55.007* -30.4 0.019 L1 fixed double 04KU 5144 078G 28.939 7250 0.014 Iono free fixed 077G 28.947 -0.8 0.014 Iono free fixed 076G 28.940 -0.1 0.020 Iono free fixed 5144 ZINC 078A -33.045 6167 0.011 Iono free fixed 077A -33.051 -0.6 0.009 Iono free fixed 076A -33.063 -1.8 0.013 Iono free fixed Differences in GPS-derived height (dU) values between repeat baselines. BR13 to BR14 – float solution; RMS value is low but unable to fix integers. Try to reprocess or re-observe this baseline. Note that the second and third baseline comparison is very close. BR14 to GR15 – high RMS on all three baselines; obstructions, multipath? BR19 to CL20 – high RMS on all three baselines. Note the second and third baselines are much closer together. Reprocess all baselines that have repeat differences greater than 2 cm. *NOTE - Reprocess all vectors which have difference greater than 2 cm.

Guidelines for Establishing GPS-Derived Orthometric Heights (Standards: 2 cm and 5 cm) http://www.ngs.noaa.gov/ SEARCH: “NGS 59” Guidelines - will eventually become routine; not so much explanation of why its done but provides background information. Repeat baselines, station spacing, fixed height antenna setups, identify and control all error sources, tie local networks together, etc.. GPS-Derived Orthometric Heights Guidelines will build from the NOS NGS-58 GPS-Derived Ellipsoid Heights Guidelines. Similar format, many similar appendices, etc.

A Guide for Establishing GPS-Derived Orthometric Heights (Standards: 2 cm and 5 cm) 3-4-5 System Three Basic Rules Four Basic Control Requirements Five Basic Procedures General idea of guidelines.

Three Basic Rules Rule 1: Rule 2: Rule 3: Follow NGS’ guidelines for establishing GPS-derived ellipsoid heights (NGS 58:Standards: 2 cm and 5 cm) Rule 2: Use latest National Geoid Model, i.e., GEOID09 Rule 3: Use latest National Vertical Datum, i.e., NAVD 88 Pretty straight forward.

FOUR BASIC CONTROL REQUIREMENTS BCR-1: Occupy stations with known NAVD 88 orthometric heights Stations should be evenly distributed throughout project BCR-2: Project areas less than 20 km on a side, surround project with NAVD 88 bench marks i.e., minimum number of stations is four; one in each corner of project BCR-3: Project areas greater than 20 km on a side, keep distances between GPS-occupied NAVD 88 bench marks to less than 20 km BCR-4: Projects located in mountainous regions, occupy bench marks at base and summit of mountains, even if distance is less than 20 km This same information is required by the GPS-derived Orthometric heights guidelines. Producing ellipsoid heights at the 2 cm level of accuracy is essential to producing accurate GPS-derived orthometric heights. Surrounding project areas will be difficult in many instances with existing NAVD88 bench marks. Level ties may be necessary to provide this requirement. Extra bench marks allow independent analysis through the adjustment process and may help identify potential bench marks with questionable stability or poor published elevations. Bench marks at base and top of mountains may help identify/rectify issues that are not otherwise apparent in the geoid model.

BCR Example BCR1: Sketch indicates that the 20 km rule was met. Sample GPS-derived orthometric height project illustrating conditions of basic control requirements. NAVD88 bench marks shown as yellow boxes. Project area is larger than 20 km so minimum 20 km spacing between bench marks is required. Bench marks are scattered throughout and surround project area. Circled bench marks are required to satisfy guidelines. Additional bench marks provide redundancy, allow checks on adjusted elevations, and could be alternate marks if one or more of the circled mark’s published elevations don’t fit the survey. BCR2: This requirement is not applicable because the project is greater than 20 km on a side. BCR3: Circled bench marks are mandatory. Analysis must indicate bench marks have valid NAVD 88 heights. Other BMs can be substituted but user must adhere to 20 km requirement. BCR4: This requirement is not applicable because project is not in a mountainous region.

FAIRFAX COUNTY VERTICALS USED 40 KM

Five Basic Procedures BP-1: Perform 3-D minimum-constraint least squares adjustment of GPS survey project Constrain 1 latitude, 1 longitude, 1 orthometric height (Recall that ellipsoid heights have already been analyzed and adjusted) BP-2: Analyze adjustment results from BP-1 Detect and remove all data outliers The basic procedures describe the network adjustment and analysis routine. You are starting with good GPS-derived ellipsoid heights. If you don’t achieve 2 cm ellipsoid heights you can’t expect 2 cm orthometric heights. Constraining one orthometric height shifts the adjusted elevations from ellipsoid height plus geoid height to the orthometric “plane.” This provides a chance to compare the adjusted GPS-derived orthometric heights with their published orthometric elevations.

Plot of free adjustment ellipsoid height residuals by baseline length. Identify and investigate residuals greater than 2 cm. Reprocess if possible, re-observe if necessary. After performing minimum constraint adjustment, plot ellipsoid height residuals (or dU residuals) and investigate all residuals greater than 2 cm.

Repeat baseline comparisons also indicate which baselines will show large residuals. Station pairs with large residuals, i.e., greater than 2.5 cm, also have large repeat base line differences. NGS guidelines for estimating GPS-derived ellipsoid heights require user to re-observe these base lines. Following NGS guidelines provides enough redundancy for adjustment process to detect outliers and apply residual on appropriate observation, i.e., the bad vector.

Five Basic Procedures (continued) BP-3: Compute differences between GPS-derived orthometric heights from minimum-constraint adjustment in BP-2 and published NAVD88 orthometric heights for all known bench marks Compare free adjusted NAVD88 elevations against published elevations. Determine trends from differences, such as slope, and which bench marks don’t fit. Check monument type and setting information, reported station condition, and history data (i.e., mark set and leveled in 1950) to see if there is a reason for station elevation to not fit with the adjustment.

CONTROL COMPARISON OUTLIERS? PASSIVE CONTROL QUALITY (OVER TIME) GEOID MODEL QUALITY

GPS-Derived Orthometric Heights Minus NAVD88 Heights Geoid99 Units = Centimeters Plot height differences on map to show trends, such as slope, and note relationship of adjacent stations. A slope or tilt across the project area is expected but large jumps between adjacent stations should not happen. All height differences are under 5 cm and most are less than 2 cm. Almost all relative height differences between adjacent station pairs are less than 2 cm. However, most of the height differences appear to be positive relative to the southwest corner of the project.

Five Basic Procedures (continued) BP-4: Determine which BMs have valid NAVD88 height values from results from BP-3 Differences need to agree 2 cm for 2 cm survey Differences need to agree 5 cm for 5 cm survey May detect systematic tilt over large areas Solve for geoidal slope and scale BP-5: Perform constrained adjustment with results from BP-4 Constrain 1 latitude, 1 longitude, all valid orthometric height values Ensure final heights not distorted in adjustment Constraining the selected bench marks at the 20 km spacing and comparing how the NAVD88 orthometric heights fit at the rest of the bench marks provides an excellent check on the validity of the adjusted network. The final constrained vertical adjustment includes all orthometric heights at bench marks determined to be consistent with the rest of the network. After final constrained adjustment has been performed make sure that the constraints didn’t distort the rest of the adjustment.

GPS-Derived Orthometric Heights Minus NAVD88 Heights Geoid99 Units = Centimeters Plot height differences on map after tilted plane has been removed and note relationships especially between of adjacent stations. The station in the far northwest corner of the project seems to be the only one that falls outside of 2 cm. This was one of the originally chosen 20 km stations. The “extra” stations allows this one to be removed as a constraint yet provide NAVD88 bench marks at the 20 km spacing. To detect and remove any systematic trend, a tilted plane is best fit to the height differences (Vincenty 1987, Zilkoski and Hothem 1989). After a trend has been removed, all the differences are less than +/- 2 cm except for one and almost all relative differences between adjacent stations are less than 2 cm.

GPS-Derived Orthometric Heights Minus NAVD88 Heights Geoid99 Units = Centimeters Plot height differences on map after tilted plane and the one rejected NAVD88 constraint station has been removed. All remaining constrained stations show that this project is within the 2 cm standard. The NAVD88 orthometric heights determined for the remaining stations included in this network should reflect the same differences as the constrained stations surrounding them. After rejecting the largest height difference (-2.4 cm), of all the closely spaced station pairs only 3 are greater than 2 cm, 1 is greater than 2.5 cm and none are greater than 3 cm.

The red stars are the CORS used in the "Height Mod" projects. Following NGS guidelines, achieving 2 cm or 5 cm accuracy, and submitting the project to NGS will get NAVD88 orthometric elevations published to centimeters. Metadata will define the method of determination as that done by GPS observations using a high resolution geoid model using precise GPS observations and processing techniques. The stations in blue designated as "Height Mod" in the NGS Integrated Database

Identified as Height Mod survey station Elevation published to centimeters Following NGS guidelines, achieving 2 cm or 5 cm accuracy, and submitting the project to NGS will get NAVD88 orthometric elevations published to centimeters. Metadata will define the method of determination as that done by GPS observations using a high resolution geoid model using precise GPS observations and processing techniques. Orthometric height determined by GPS

GPS-Derived Heights from GEOID03 Separation Topography A B C D E F Hh-N Ellipsoid h GEOID03 N = Published NAVD88 Orthometric Height = New Control

Constrained Vertical Adjustment Ellipsoid Height Adjusted to Fit Constrained Orthometric Heights GPS-Derived Orthometric Heights Topography A B C D E F H Geoid based on Ortho Heights Ellipsoid h HGPS hadj Adjusted Ellipsoid N GEOID03 GEOID03 = Published NAVD88 Orthometric Height = New Control

ADJUSTMENT TO PASSIVE CONTROL

Combined Network Vertical Adjustment 3-D Vertical Adjustment (Orthometric Heights) Minimum Constrained [One fixed station] Fix latitude, longitude, and orthometric height at one station Resolve all blunders and large residuals Compare orthometric heights from adjustment with published NAVD 88 Determine which NAVD 88 bench marks should be fixed Constrained [All suitable orthometric heights fixed] Fix latitude, longitude at one station Fix orthometric heights at all suitable stations Make sure the constraints did not distort the project

Minimally Constrained Vertical Adjustment CORS HARN NAVD’88 BM New Station 121°40’W 122°35’W 37°50’N 38°20’N LATITUDE LONGITUDE Spacing Station Primary Base Station 8.2km Constrained Vertically 1 horizontal latitude and longitude 1 NAVD88 orthometric heights

Constrained Vertical Adjustment CORS HARN NAVD’88 BM New Station 121°40’W 122°35’W 37°50’N 38°20’N LATITUDE LONGITUDE Spacing Station Primary Base Station 8.2km Constrained Vertically 1 horizontal latitude and longitude All valid NAVD88 orthometric heights

Published NAVD88 to GPS Derived NGS Data Sheet - GEOID03 Published NAVD88 to GPS Derived H = h - N 102.431 = 69.78 - (-32.60) 102.431  102.38 102.429! GEOID 09 Typical published NGS data sheet with current coordinate and height information. This is one of the bench marks observed with GPS in the San Francisco Bay Demonstration Project. GPS 2 cm heights determined on a NAVD88 bench mark. The published NAVD88 orthometric height does not equal the published ellipsoid height minus the geoid height as indicated by the equation H=h-N. These heights were determined by two entirely different methods, GPS-derived orthometric heights (ellipsoid heights minus geoid heights) and leveling-derived orthometric heights (optical leveling plus orthometric corrections), not to mention bench mark stability, and reflects our uncertainty of each. As our understanding and models improve this disparity should become smaller, as indicated between the various geoid models, but will never really equal. GEOID96 = 0.17 m GEOID99 = 0.11 m GEOID03 = 0.05 m GEOID 09 = 0.002 m

SUMMARY Mistakes and systematic errors must be removed before the adjustment A least squares adjustment handles random errors and provides a single solution (Try to eliminate all systematic errors) The Minimally Constrained adjustment checks the internal consistency of the network The Constrained adjustment checks the existing control and references the network to the datum The vertical adjustment estimates GPS-derived Orthometric heights- Approaching 3rd order leveling accuracies

ORTHOMETRIC HEIGHTS FROM REAL TIME GNSS POSITIONING NAD 83 ELLIPSOID + NGS HYBRID GEOID MODEL OR LOCK TO PASSIVE MONUMENTATION (INCLINED PLANE + NGS HYBRID GEOID MODEL) OR DIFFERENCE IN ELLIPSOID HEIGHTS FOR ≤ 10 Km

REAL TIME GNSS POSITIONING USERS GUIDELINES

>200 RTN WORLD WIDE >80 RTN IN USA 35+ STATE DOT MANY COOP EFFORTS

SUMMARY TABLE FROM THE SINGLE-BASE GUIDELINES

BEST METHODS FROM THE GUIDELINES: THE 7 “C’S” of NOAA’s NGS http://www.ngs.noaa.gov/PUBS_LIB/NGSRealTimeUserGuidelines.v2.0.4.pdf http://www.ngs.noaa.gov/ SEARCH: “CLASSICAL REAL TIME” CHECK EQUIPMENT COMMUNICATION CONDITIONS CALIBRATION (OR NOT) COORDINATES COLLECTION CONFIDENCE THE CONTROL IS AT THE POLE

METADATA BESIDES ATTRIBUTE FIELDS, THE RT PRACTICIONER MUST KEEP RECORDS OF ITEMS NOT RECORDED IN THE FIELD, FOR INSTANCE: WHAT IS THE SOURCE OF THE DATA? WHAT IS THE DATUM/ADJUSTMENT/EPOCH? WHAT ARE THE FIELD CONDITIONS? WHAT EQUIPMENT WAS USED, ESPECIALLY- WHAT ANTENNA? WHAT FIRMWARE WAS IN THE RECEIVER & COLLECTOR? WHAT REDUNDANCY, IF ANY, WAS USED?

QUICK FIELD SUMMARY: Set the base at a wide open site Set rover elevation mask between 12° & 15° The more satellites the better The lower the PDOP the better The more redundancy the better Beware multipath Beware long initialization times Beware antenna height blunders Survey with “fixed” solutions only Always check known points before, during and after new location sessions Keep equipment adjusted for highest accuracy Communication should be continuous while locating a point Precision displayed in the data collector can be at the 68 percent confidence level, which is only about half the error spread to get 95 percent confidence Have back up batteries & cables RT doesn’t like tree canopy or tall buildings

THE QUICK SUMMARY BOILED DOWN: COMMUNICATIONS: THE KEY TO SUCCESS CHECK SHOT: FIRST BEFORE NEW WORK REDUNDANCY: FOR CONFIDENCE

CONSTRAINING PASSIVE MARKS

CALIBRATIONS/VERTICAL LOCALIZATIONS

USING OPUS-S OR OPUS –RS WITH REAL TIME POSITIONING FOR SMALL PROJECTS

GNSS DERIVED HEIGHTS Summary of expected orthometric precisions/accuracies REMEMBER REDUNDANCY AND A CHECK ON KNOWN POINTS CORS = 0.05 m OPUS-S = 0.05 m OPUS-RS = 0.05 m NGS 58/59 = 0.02 m local, 0.05 m to NSRS SINGLE BASE REAL TIME = 0.02 m ≤ 10 Km, remember GIGO RTN = 0.03- 0.05 m,