ACCURACY ACCELERATED A PIONEERING NEW APPROACH IN DENMARK IS BOOSTING PRODUCTIVITY IN MAINTAINING PRECISE VERTICAL DATA. JOHN STENMARK REPORTS For such a flat country, Denmark is surprisingly interested in elevations – and for good reason. The low-lying nation is made up of a peninsula and an archipelago of large and small islands surrounded by the Baltic Sea. The country’s mean elevation is roughly 34m and much of its terrain is flat plains. As a result, Denmark is particularly vulnerable to the effects of rising sea levels. The concern over elevations is driving an innovative application of geospatial technologies. Work is underway in Denmark to modernise the collection and maintenance of geodetic heighting information. Having accurate, up-to- date data is crucial, especially in the low areas of the country where rising water levels may affect dams, transport infrastructure and agriculture. The work to gather and maintain data on Denmark’s network of vertical control benchmarks falls to the Danish Agency of Data Supply and Efficiency (SDFE). For years, SDFE has used a combination of conventional geometric and trigonometric levelling to maintain the control points. Recently, the agency began an effort to check benchmarks over the entire nation. To meet the technical and budget requirements, surveyors needed to develop an approach that would increase measuring efficiency while preserving precision and accuracy. The new approach combines motorised trigonometric levelling (MTL) with new total station technology and customised software. The experience gained from work on initial projects has produced encouraging results. Under MTL, surveying total stations and prism targets are installed in two vehicles and driven from point to point. The vehicles use leapfrog techniques along a route conducting reciprocal measurements. The reciprocal measurements enable analysts to remove systematic error due to refraction and account for small errors to produce precise vertical differences between the two points. Specially designed mounting systems ensure that the equipment is stable whenever measurements are in progress. The MTL procedures used by SDFE call for multiple observations in Face 1 and Face 2 positions. The process produces very good accuracy and is much faster than walking and using tripod-mounted instruments and targets. For projects that require second-order precision, motorised trigonometric levelling is an attractive option. After analysing the MTL process, geospatial specialists at Copenhagen-based Geoteam A/S realised that much of the field time consisted of physically operating the total station to obtain multiple measurements. They found that by using robotic total stations to make 26 June 2017 | GeoConnexion International Magazine
Transcript
ACCURACY ACCELERATEDA PIONEERING NEW APPROACH IN DENMARK IS BOOSTING PRODUCTIVITYIN MAINTAINING PRECISE VERTICAL DATA. JOHN STENMARK REPORTS
For such a flat country, Denmark is surprisingly interested in elevations– and for good reason. The low-lying nation is made up of a peninsulaand an archipelago of large and small islands surrounded by the BalticSea. The country’s mean elevation is roughly 34m and much of itsterrain is flat plains. As a result, Denmark is particularly vulnerable tothe effects of rising sea levels. The concern over elevations is driving aninnovative application of geospatial technologies.
Work is underway in Denmark to modernise the collection andmaintenance of geodetic heighting information. Having accurate, up-to-date data is crucial, especially in the low areas of the country where risingwater levels may affect dams, transport infrastructure and agriculture.
The work to gather and maintain data on Denmark’s networkof vertical control benchmarks falls to the Danish Agency ofData Supply and Efficiency (SDFE). For years, SDFE has used acombination of conventional geometric and trigonometric levellingto maintain the control points. Recently, the agency began aneffort to check benchmarks over the entire nation. To meet thetechnical and budget requirements, surveyors needed to developan approach that would increase measuring efficiency whilepreserving precision and accuracy. The new approach combines
motorised trigonometric levelling (MTL) with new total stationtechnology and customised software. The experience gained fromwork on initial projects has produced encouraging results.
Under MTL, surveying total stations and prism targets areinstalled in two vehicles and driven from point to point. The vehiclesuse leapfrog techniques along a route conducting reciprocalmeasurements. The reciprocal measurements enable analysts toremove systematic error due to refraction and account for smallerrors to produce precise vertical differences between the twopoints. Specially designed mounting systems ensure that theequipment is stable whenever measurements are in progress.
The MTL procedures used by SDFE call for multiple observationsin Face 1 and Face 2 positions. The process produces very goodaccuracy and is much faster than walking and using tripod-mountedinstruments and targets. For projects that require second-orderprecision, motorised trigonometric levelling is an attractive option.
After analysing the MTL process, geospatial specialists atCopenhagen-based Geoteam A/S realised that much of the field timeconsisted of physically operating the total station to obtain multiplemeasurements. They found that by using robotic total stations to make
26 June 2017 | GeoConnexion InternationalMagazine
SURVEYING
The laptop software collects and displays measurements from both total stations. The solution processesthe data and automatically flags observations that don’t meet specifications. Operators can take additionalmeasurements and avoid revisits to correct errors
A survey vehicle equipped for motorized triglevelling. The high position of the instrumentprovides flexibility in selecting locations forobservations. The instrument can be lowered androof retracted when not in use for measurements
Surveyors can operate the vehicle andmeasurements from the driver’s seat. Customsoftware controls both instruments and enablesoperators to verify results before moving to the nextset up
the observations, they could significantlyreduce the time needed for the survey.
Using a robotic instrument to perform thesightings instead of manually-operated totalstations increases the accuracy immediately.When taken over a full day of observing, anobserver’s performance will be affected byweather, distractions and fatigue. By contrast,the performance of a robotic instrumentis much more stable and is not affected bytime, lighting and environmental conditions.
“The speed of the observations willincrease substantially,” says Geoteamdirector Henrik Johansen. “The time savingsin the observation phase should increasethe accuracy by minimising the influenceof fluctuating refraction. Alternatively,the saved time could also be used tomeasure more rounds, which would alsoincrease the accuracy and precision.”
New designs for productivityWorking closely with SDFE, Geoteam specialistsdeveloped an approach that would integraterobotic total stations into the SDFE operations.The basics of the solution were formulated tofollow very strict requirements and based onmany years of experience. To meet accuracyspecifications, Geoteam used Trimble S9HP total stations with 0.5'' angular precisionand robotic operation. The instruments areequipped with Trimble Vision technology,which enables operators to see what theinstrument is seeing through its telescope onthe laptop controlling the surveys.
Additionally, Geoteam was invited tosuggest improvements to SDFE’s existingprocedures. For example, they suggestedmounting a small prism on top of theinstruments for tie in between instrumentstations. The concept provided secure positionand orientation of the prism for precisemeasurements. It enables rapid reciprocalmeasurements between two instrumentstations and eliminates the need to removeor reposition prism targets. SDFE fittedcustom-built tripods into the vehicles; themotorised tripods extend down throughholes in the floor to provide a stable platformfor the high-precision total stations. Thetripods retract when the vehicles are readyto move to the next observing station.
To conduct operations in the field,Geoteam programmers used the TrimblePrecise SDK (software development kit)to create specialised software to run on alaptop in one of the vehicles. Communicationbetween the two survey vehicles is doneover a mobile LAN using dedicated routerswith a radio backup system. Additionally,the PCs are connected to the Internet tomake it possible to use remote desktopsoftware to manage the software andhandle operations in both vehicles.
The laptop software controls bothinstruments to conduct measurementsand then to store and analyse the data. The
software provides measurement optionsand quality control on the spot to ensurecrews meet the required accuracy of theproject. It controls the needed functionsof the instrument as well as temperaturemeasurements, ppm calculations,averaging the rounds, calculating heightdifferences, accumulated height differencesand accumulated distances. Operatorsin the field can inspect the results,remove questionable measurements oradd measurements as needed to meetthe defined criteria for precision.
Combining accuracy and productivityAs Geoteam developed the system, theyworked with SDFE to conduct a series of teststo better understand the performance andoutput. In early tests using a one-secondTrimble S8 total station, they measured twoknown level lines using MTL and comparedthe results to existing data. The tests producedaccuracy of roughly 1mm/km.
Johansen says that subsequent testingusing the Trimble S9 HP instruments over a
36km course showed an error of 0.9 mm/km. “These results are very impressive,”hesays. “Further investigation of temperaturemeasurements, number of rounds andoperational experience indicates that thisresult can be even better. The speed ofmeasurement and elimination of operatorerror and fatigue will enable the users to takemore data and produce higher accuracy.”
The value that the system providesextends well beyond its accuracy. Johansensays that the new approach is proving tobe very fast. As crews gain experience withthe motorised techniques and software, heexpects that they will be able to performprecise trig levelling at a rate of two tothree kilometres per hour. The savingsstem from the faster operation togetherwith the ability to make fewer setups overa given route than using conventionallevelling. At the same time, crew sizes canbe cut from four people down to two.
Taken together, the changes translateinto expected cost savings of 35%compared to current techniques.
www.geoconnexion.com 27
Testing routes for the instruments and software. Levelling routes were surveyed twice (blue) and the resultwas compared to a precise geometric route (red). Another route (green) was used to test the distance meterand prisms.
A total station and top-mounted prism assemblyfor reciprocal measurements. The prism rides in arotatable bearing to provide stable orientation whilethe total station turns beneath it
A total station installed in the survey vehicles.Custom-built tripods provide stable mounts and theability to extend and retract as needed.
The solution also offers advantages insafety and convenience. Operators rarelyneed to leave their vehicles to work in traffic,and setup locations can be chosen withsafety and visibility in mind. Because theinstruments are installed in a high positionin the vehicle, line of sight is improved,which creates more flexible operation. Thevehicles can be parked in locations notsuitable for survey crews working on footand with standard ground-mounted tripods.The public can travel the roads safely andwith little disruption; in many cases thepassing cars may not be aware of the precisemeasurements taking place just above theirheads. The higher instrument positions alsohelp mitigate errors due to refraction.
Looking ahead, Geoteam and SDFEalso hope that MTL methods can becomean adequate substitute for motorisedgeometric levelling using conventional levelinstruments – provided that the accuracyreaches the required levels. There are alreadyindications that this is an achievable goal:
A NOT-SO-NEW SOLUTION
Conventional geometric levelling useshigh-precision levels and invar staffs toconduct precise measurements. Theapproach provides excellent accuracy,but is constrained by differences interrain and limited lengths of sightlines. As a result, geometric levelling isexpensive. Motorised geometric levellinguses vehicles to move instrumentsand targets quickly along a levellingroute. The vehicles simply reduce thetime to move from point to point,which helps to reduce the costs.
Trigonometric levelling is a viable– if not widely used – alternative togeometric levelling. With sufficientcare, it’s possible to measure longerlines and larger height differenceswith at least second-order accuracy.
Trig levelling is not a new concept.Soon after the introduction of the firsthigh-precision integrated total stations inthe early 1980s, Adam Chrzanowski of theUniversity of New Brunswick tested thetechnology. He showed that trig levellingcould produce accuracy of 2mm√km (atthe one-sigma level) with sight lines aslong as 300m and at a speed compatiblewith conventional geometric levelling. Thetechnique could prove to be especiallyvaluable in hilly or mountainous terrain.
Trig levelling was a featured topic atthe 1985 International Symposium on theNorth American Vertical Datum and againat the 1986 FIG Congress, where a reportfrom the National Land Survey of Swedenalso demonstrated good results. And theSwedes added a twist – they used vehicles.It was one of the first demonstrationsof motorisation for trig levelling.
In a 1989 paper, Chrzanowski providea detailed description of motorisedtrig levelling techniques to producehigh-quality results. Chrzanowski usedhalf-second total stations mountedin pick-up trucks and tested leapfrogtechniques and reciprocal measurements.He obtained accuracy of between 1mm/km and 2mm/km and concluded that triglevelling could be a logical replacementfor conventional geometric levelling.
Chrzanowski found that accuracyof measurement of vertical angleswas an important factor in the overallachievable accuracy. Angular accuracycould be improved by taking moreobservations at each station. But hepointed out that to do so using themanually operated total stations availableat the time increased observationtime to the point that trig levellingwould be rendered uneconomical. Theexperience in Denmark with robotic totalstations has removed this concern.
Johansen says that survey crews usingthe new solution have delivered accuracyresult in the range of 0.6mm/km.
The efforts of the Danish teamsare helping to expand and cement therole of trig levelling. The use of precisetotal stations driven by well-designedsoftware can transform the work ofestablishing and maintaining networksof high-accuracy vertical control.
THE EFFORTS OF THEDANISH TEAMS AREHELPING TO EXPANDAND CEMENT THE ROLEOF TRIG LEVELLING
John Stenmark is a writer andconsultant working in the geospatialand associated industries
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