The sight distances were measured using Rollfix measuring wheels. Along rail-ways a track trail was mounted to keep a measuring wheel on a rail. The distances outside railways and roads were measured using steel or plastic measuring tapes, especially when spirit levels were used. Although digital levels can measure dis-tances, measuring wheels were still used.
During the back measurements, the distances of bench mark intervals were measured and the locations of remarkable objects, such us bridges and junctions, were determined. This information was used in bench mark descriptions and in the bench mark list [1].
The bench marks’ side distances were measured from the middle of the road
to the bench mark position using a tape measure.
Chapter 4
The description of the field work
There were an observer and four or five crew members in the expedition. Only four crew members were needed when the Zeiss Ni002 or the digital level DiNi12 was used (Figure 4.1). The Zeiss Ni002 level is waterproof and not sensitive to sunshine, but umbrellas were used with the other instruments. Before record-ing the digital levels, the record keepers were required to store observations in note books or on handheld computers. The record keepers carried thermometer holders which were equipped with a small table.
Bench mark intervals are measured back and forth and the height difference is the mean of the measurements. The index error of rods is removed by observing an even number of setups. If an odd number of setups is observed, then the other rod is changed onto the bench mark before the back measurement. The observing and rod positions are marked during the fore measurement. The closing measurements are performed using the same positions. During the measurements on roads traffic signs warn any oncoming traffic. On railways, safety personnel from the railway company took care of safety. Before the measuring work was started, the old bench marks had to be restored and the new ones established
Figure 4.1: Levelling measurement using the digital level DiNi12. The distance
measurer carries the thermometer holder (Photo M. Poutanen).
Figure 4.2: The device, between a hammer and a bolt, was used to protect the bolts during pounding (Photo M. Takalo).
where it was considered necessary.
4.1 Maintenance of the levelling network
Most levelling routes followed the old levelling lines of the Second Levelling. New bench marks were established if the distance between the successive bench marks was too long. Bedrock bench marks are used in deformation and land uplift studies so, if possible, new bench marks were established on bedrock.
The bench mark bolts are 15 cm long and their arm is 22 mm thick. Before 1987, the arm was 25 mm thick. The diameter of the bolt’s spherical head is 38 mm and in a bolt there is a slit for a wedge (Figure 4.2).
In the beginning of the levelling, boreholes for bench marks were drilled using Cobra rock drilling machines, later on Torna electric hammers were used. Since 1987, the drilling was performed using gasoline-powered rotary hammers: a Part-ner (1987), a Kawasaki Ten 22 (1988–1997) and a Ryobi ER-382 (1998–2006).
Soldering concrete was used to strengthen the fastening of the bolts and prevent-ing the flow of water into boreholes. Bolts were painted usprevent-ing anticorrosive paint (Figure 4.3).
The bench mark identifier is a five-digit number – two places are for the setting year, one number for the surveyor and two numbers for the annual serial number of bench marks. In the Second Levelling and in the beginning of the Third Levelling, the bench mark identifiers were engraved using hammers and chisels, but later the work was done using drilling machines.
The bench mark descriptions were done for both old and new bench marks,
and include at least identifiers and coordinates. The site location is bedrock,
boulder, bridge, culvert, or foundation. Side distances were measured from the
middle of the road. The approximate height of the bench mark relative to the road
surface was determined with the aid of a Suunto clinometer. The levelling routes
went along asphalt and dirt roads. On railways, the surface material was mostly
gravel. In the beginning of the levelling, the bench mark locations and coordinates
were determined using topographic maps. Since 2001, coordinates have been
measured with handheld GPS receivers, which typically have an accuracy of some
metres.
Figure 4.3: In the making of a new bench mark near Toivala in 2005. Three phases of work are drilling, pounding and painting of the bench mark (Photos I.
and P.Lehmuskoski).
4.2 On the weather conditions for the levelling work
Daily measurements were usually performed in two parts. Typically two bench mark intervals were measured in a day. The first measurements were started about two hours after sunrise and the work continued after the midday break. In the evening, the measurements were stopped one hour before sunset at the latest.
On rainy days and especially in the late autumn, the levelling expeditions worked continuously without any midday break. In Lapland, the daily measurements were performed in one session.
Overcast and rainy days are optimal for levelling work. In ideal weather con-ditions, the ground level temperature (measured at 0.5 m) should be slightly higher than the air temperature at 2.5 m above the ground. In other words, the temperature gradient should be from -0.1
◦C to -0.5
◦C. If the negative gradient was greater, then shorter sight distances were used to decrease short-period shim-mering. During the night and after sunrise there is a danger of slow vertical air movements which are clearly visible with the naked eye.
Shorter sight distances were used on sunny days. A maximum sighting dis-tance of 45 m was used with DiNi12 levels. With the previous levels, the maxi-mum sight distance of 55 m was used. The sight distances from the instrument to the rods have to be equal as this reduces the errors due to collimation, refrac-tion and curvature of the Earth. With a digital level, the cumulative distance difference between the back and forward directions was possible to check and correct during the measurements. The recommendation is that sight lines should be more than 0.5 m above the ground to reduce the refraction effect.
The measurements were performed mainly in spring and autumn. In July, the levelling expeditions had a summer break in Southern and Central Finland.
In Northern Finland, the levelling season started in June and it was continued to
September or October.
Figure 4.4: Bicycle levelling in Hyvink¨ a¨ a in 1979 (Photo S. Kora).
4.3 Movement of the expeditions
Traditionally, Finnish levelling expeditions have moved on foot, but during the early years of the levelling bicycles on roads and handcars on railways were used.
This choice was based on the test measurements during the re-levelling of Lapland in 1972–1975 [14]. Motorized levelling was not used in Finland, although it was used widely in the other Nordic countries [26].
Bicycles and handcars were used in 1978–1985, but when the automatic Zeiss Ni002 levels were changed to spirit levels, they were abandoned. The Zeiss Ni002 was used with vehicles, because its rotating ocular allowed observations from one observing position,
In the bicycle method (Figure 4.4), one bicycle measured distances and trans-ported the rod base spikes and their pounding device. The record keeper’s bicycle had a table and a differential thermometer rack while the observer’s bicycle had a rack for the instrument.
In the handcar method (Figure 4.5), the rods kept their mutual order from the start to the middle of the bench mark intervals, where the rods were changed between handcars. This procedure eliminated the impact of zero point differences between the rod scales. As a whole, the rods were in the back and fore rod positions at equal times. In the normal levelling, the back rod is moved to the fore position after every setup.
The observers and record keepers were in the same handcar, which was equipped with a table and racks for a tripod and a differential thermometer.
For the recordings, the tripods were taken out from handcars. One person moved
on foot and measured places for the instruments and rods while the other crew
members and equipment were located on handcars.
Figure 4.5: Handcar levelling in Inkeroinen in 1979 (Photo P. Lehmuskoski).
Figure 4.6: The adjustment of the Wild N3 using the Kukkam¨ aki method in
1996. The record keeper observes the air temperature difference and writes down
the rod readings (Photo I. Syv¨ anper¨ a).
4.4 Collimation error of the instrument
The collimation error i.e. the deviation between the instrument’s line of sight and horizontal plane was determined once a week using the Kukkam¨ aki method.
In the method, the difference in height is measured in two locations. First, an instrument is placed in the middle of the rods and the sight distance is 10 m.
Second, the instrument is outside the rods, so that the distances to the rods are 20 m and 40 m (Figure 4.6). Due to the unequal sight distances, the line of the sight’s deviation from the horizontal level can be computed. The largest accepted error was 0.02 mm/m.
The collimation error of the Zeiss NiA was corrected by adjusting the main level and the Wild N3 was adjusted by turning the wedge-shaped cover glass in front of the objective. The determination of the collimation error was repeated and corrected until the error was below the threshold.
The collimation errors of the Zeiss Ni002 and digital levels were treated dif-ferently. The automatic level Zeiss Ni002 had to be sent to a service, if the collimation error was too large. The digital levels were able to correct the col-limation errors. The levels saved the colcol-limation error and corrected the rod readings. Normally errors were in range from -10” to +10”, but there was one case, when the increased error was more than 100”.
4.5 Rejection limits for the bench mark intervals and setups
The standard deviation m of the double run levelling observation is m = ∆(mm)
2 p
L(km) . (4.1)
In the formula, ∆ (mm) is the difference between the back and forth mea-surements and L (km) is the length of the bench mark interval. The unit of the standard deviation m is mm/ √
km.
Since the late 1980s, the maximum accepted difference between the back and forth measurements was 2 √
L mm, which is a standard deviation of ±1.0 mm/ √
km. In the beginning of the Third Levelling, the limit was 1.6 √
L. If the bench mark interval had to be measured for the second time, both directions were measured.
A heuristic approach for rejection limits was applied with Zeiss DiNi12 levels, if they had systemically large differences between the back and forth measure-ments. As a rule of thumb, the observations were accepted if the difference fulfilled the rejection criterion, after removing an average systematic difference.
At setups, the four rod readings were observed. The observing procedure was B1, F1, F2, B2, where B stands for the reading from the back rod and F from the fore rod. The rejection criterion was the difference of (B1-F1) and (B2-F2).
The maximum accepted difference was 0.30 mm. In 1989-2000, the threshold of
0.45 mm was used.
4.6 Data processing
Rod readings, sight distances, air temperature gradients, and information of rain and intensity of the sun were recorded at every setup. Other weather parameters were recorded three times during the measurements. Short-period shimmering (turbulence) of air, cloud cover and wind speed (m/s), were estimated by the observers. In addition to these factors, the air temperature was measured and recorded.
On railways, passing trains were recorded. A train went before a setup or in the middle of a setup. In the latter case two first rod readings were recorded before the train and the observation was continued after the passing of the train.
This information was more important when rail nails or unreliable rail clamps were used.
In the beginning of the levelling, observations were written down in note-books. Later handheld Husky Hunter computers replaced notenote-books. The first data collecting program was run on a CP/M operating system [27]. In 1987-1990, daily observations were copied to floppy disks using Bondwell computers and then Husky Oracle floppy disc drivers were used from 1991. Observations, tempera-ture values and the recorded weather notes were combined into measurement documents, which were printed daily.
Digital levels record observations into PC Cards. Following the daily mea-surements, the content of the PC Card was copied to computers and to floppy discs or USB flash drives. The data from the temperature logger Fluke 54II was transferred using an infrared link. The observers recorded weather information into notebooks.
The documents of corrected observations i.e. line papers (Figure 4.7) were computed after field seasons. In these documents, all corrections are presented for both directions (“A” is a direction of a line and “B” is a closing measurement).
Other columns include corrected height differences, differences between fore and back measurements, gravity at bench marks and geopotential differences. The program computes the epoch of levelling and standard deviations.
The computation program collects data from the observation documents,
reads gravity values at the bench marks, computes corrections and presents
ob-servations in metric and geopotential differences. Over the years, several
com-puting programs were used. There are no major differences between the program
versions, which were LPAP71 (1969-1977), LPUS93U (1986-1994), and LPAP98
(since 1994). The programming language was FORTRAN 66 and 77. All the
aforementioned computation programs computed observations relative to a mean
tidal geoid. The heights and height differencies were transformed to a zero tidal
system after adjustments.
Figure 4.7: The computation document of the levelling line Kelv¨ a-Tiensuu, which
was measured in 1992.
Chapter 5
Rod comparators
The length changes of rod scales have a direct impact for rod readings and thus for height differences. In rod calibrations a linear coefficient (µm/m) for the rod’s scale at 20
◦C and a thermal expansion coefficient (µm/m)/
◦C are determined.
In the rod scale calibrations, the true positions of graduation lines are measured using a laser interferometer. The rods are moved along rails and the graduation lines are positioned precisely using a microscope or CCD camera. In the system calibration, the true distances are compared to the height differences which are observed by the levelling instrument. Abbe’s law has been applied in the con-struction of the comparators, i.e. the calibrated line is the direct continuation of the reference line [28].
During the Second Levelling the positions of the graduation lines were ob-served using microscopes and a steel and invar normal metre were used as length standards [4]. However, during the Third Levelling, FGI rod comparators were used. In the first version, the rods were calibrated manually in a horizontal po-sition. Later versions allowed calibrations in horizontal and vertical positions.
System calibrations of the digital levels were started in 2002.
5.1 The horizontal comparator
The first comparator was in a horizontal position on an optical bench in the FGI laboratory at Ilmala in 1974–1978. The main components were the HP 5525A laser interferometer, a retro-reflector and a microscope. The rods were shifted under the microscope using conveyers on steel rails. The calibration was performed manually. The measuring accuracy was from 2 to 3 ppm and it was dependent on the quality of graduation lines [29].
5.2 The horizontal-vertical comparator
Since 1978, calibrations have been performed in a horizontal-vertical comparator.
The prototype of the world’s first vertical laser rod comparator was designed and
tested in 1975 and it was constructed in 1978–1980. In the comparator, the
rods were moved along the 10 m horizontal and 8 m vertical wooden frames. It
was used in 1978–1994 and was housed in an unheated room in the water tower
building at Ilmala.
The laser interferometer HP 5526A was the length standard, and the mea-suring microscope was the BK 70x50 Carl Zeiss Jena. A beam splitter turned the laser beam into the vertical direction. Two guide wires kept the movement of rods parallel in relation to the laser beam. On average, the lengths of the rod scales were 3.7 µm/m shorter in a vertical position than in a horizontal position [30]. For the vertical part a lift system with a counterweight was designed.
Only five to ten percent of the graduation lines and four microscope marks were measured. The marks were engraved at the distance of one metre on the invar band. The standard deviation of the graduation line calibration was ±5 µm and in the microscope mark calibration it was ±4 µm [31]. The calibration of all graduation lines was performed once or twice during the life span of rthe ods. During that time, the thermal expansion coefficients were determined using the horizontal laser rod comparator in the Helsinki University of Technology laboratory in Otaniemi. In the unheated FGI laboratory, the determination of thermal expansion coefficients was impossible.
5.3 The rod comparator at the Masala labora-tory
FGI moved to Masala in 1995, and a new rod comparator was constructed [32].
In the new version, an HP 5529A was used as a laser interferometer, and a CCD camera COHU with a Matrox Meteor board was used instead of a measuring microscope. It had an automated weather station with a Vaisala QLI50 interface, HUMICAP MPD35 temperature and humidity sensors, and a PT100 pressure sensor. The rods were moved in a linear rigid conveyer using a stepping motor and the movement was balanced with counterweights. The comparator was controlled by Visual Basic controlling software.
In rod scale calibrations, the positions of the graduation lines were measured twice from the bottom to the top and the back at three temperatures. One cali-bration lasted about 90 min depending on the type of rod scale. The measuring accuracy of the calibration depended on the quality of the rod scale, and with 95% confidence it was between 0.7 ppm and 2.0 ppm [33]. The thermal expan-sion coefficient was based on the measurements of one graduation line interval at different temperatures. The accuracy, which was obtained from six independent measurements was approximately 0.2 (µm/m)/
◦C.
In 2002 system calibration was added to the measuring features [34]. In
sys-tem calibrations rod readings from the levelling instrument are compared to the
laser interferometer readings and thus the rod corrections include not only rod
scale information but also how well instruments interpret the scale [35].
Sys-tem calibration corrections were not utilized in the rod corrections of the Third
Levelling observations.
Chapter 6
The computation of the N2000 heights
In this chapter the computation of the Finnish levelling observations and the adjustments are presented. The selection of the EVRF2000 datum was originally based on the work done in the Working Group for Height Determination of the Nordic Geodetic Commission (NKG). Before the adjustments, the observations were corrected to the system epoch 2000.0 using the Nordic land uplift model NKG2005LU.
In 2002 the General Assembly of the Nordic Geodetic Commission (NKG) ac-cepted a resolution, which considered it desirable that the Nordic countries “adopt [height] systems with minimal differences from each other and from the European Vertical Datum”. Following the NKG proposal [36] the Technical Working Group (TWG) of the International Association of Geodesy (IAG) and its subcommission for Europe (EUREF) recommended a close co-operation between the NKG, all countries around the Baltic, the Netherlands, and the United European Levelling Network (UELN) computing centre. Subsequently, Estonia, Latvia, Lithuania, Poland, Germany and the Netherlands made their levelling data used in the EVRF2000 available to the NKG.
The N2000 height system differs only a little from its Nordic counterparts due
to the joint BLR adjustment and the inclusion of levelling lines from neighbouring
countries. Additionally, the new Swedish height system RH2000 is based on the
The N2000 height system differs only a little from its Nordic counterparts due
to the joint BLR adjustment and the inclusion of levelling lines from neighbouring
countries. Additionally, the new Swedish height system RH2000 is based on the
In document
The Third Precise Levelling of Finland
(sivua 24-0)