- 1 - Conduct aerial gravity survey countrywide for height component of the geodetic network. Draft report. submitted to

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1 - 1 - Conduct aerial gravity survey countrywide for height component of the geodetic network Draft report submitted to Ministry of Lands Housing and Human Settlements Development Kivukoni Waterfront Dar es Salaam Kgs Lyngby CVR-nr. DK December 2013 Arne Vestergaard Olesen

2 Introduction We started out with some delays. First an intentional delay in order to wait for better weather conditions. A prolonged wet season over most parts of Tanzania this year forced us to make some changes to the scheduling of the airborne survey operations. The survey was initially planned to commence by mid May but was postponed to early June. After this planned delay came some unforeseen delays. Clearances from TCAA and from the military were not in place by early June. We did a test flight on June 21 and got a good to go from the Technical Supervisor. But we had to wait until June 25 before the military released the security officer and we could commence survey operations out of Dodoma. Figure 1. Survey blocks and airports of operation: DO=Dodoma, KJ=Kilimanjaro, TG=Tanga, MT=Mtwara.

3 -3- Coastal Aviation ( provided aircraft and crew for the survey. We used Coastal s newest Cessna Grand Caravan, 5H BEE, which is equipped with the most modern avionics. This air craft was also used for the 2012 airborne gravity survey and with very satisfying results. 2. Operational overview The general survey layout is detailed in Figure 1. We started with operations out of Dodoma (DO) on June 25 to cover the southern part of Block 4 and then proceeded to Kilimanjaro (KJ) on July 2 to fin ish the northern part of Block 4 from there. Block 4 was completed on July 10. Block 3 was flown from Kilimanjaro as well. The completion of Block 3 and transfer to Tanga (TG) on July 16 marked the halfway point for the survey. Block 2 was flown from Tanga. We moved to Mtwara on July 20 and fin ished operations in block 1 on July 26. Figure 2. Ground track pattern Ph Dir Fax

4 - 4 - Figure 2 shows ground track pattern for the whole survey. All survey lines have been flown at constant altitude. The quality of airborne gravity measurement is very dependent on turbulence which means it should preferable be flown when the atmosphere is more settled. All survey lines have therefore been flown at night and in the early morning hours. Details for individual flights are listed in Appendix A. 3. Instruments Figure 3. Cabin layout in 5H BEE. To the right the LCR gravimeter and behind it the Chekan meter. The following survey equipment was installed in the aircraft: LaCoste & Romberg Air/Sea gravimeter S 38 Chekan AM gravimeter #24 Honeywell INS Javad Lexon GPS receivers (aircraft) Javad Delta GPS receivers (aircraft)

5 GPS processing and reference station coordinates. GPS reference stations were operated in all airports. These stations together with reference stations operated by SMD were used for differential trajectory determination of the aircraft. Table 1. Coordinates for airport GPS reference stations (ITRF 2008) Airport Designation Lattitude Longitude Height ell. (m) Dodoma DO Kilimanjaro KL Tanga TG Mtwara MT Coordinates for the GPS reference stations were obtained from the AUSPOS GPS service provided by Geoscience Australia. Coordinates are given in the ITRF2008 frame, see The AUSPOS service also provides estimates of the accuracy with which the coordinates are determined. These estimates in general amounted to 0.5 to 2 cm for the height component for individual sessions. Variations in the results from different sessions, typical 5 to 8 hours, were consistent with the noise estimates. Mean values for several sessions were used as final coordinates for the reference receivers, see table 1. Aircraft trajectories were computed with the WayPoint software package from NovAtel (Calgary, Canada) using precise ephemerides from International GNSS Service ( More combinations of rover and base receiver were computed for each flight and the best performing solution chosen for the final iteration of the gravity processing. Figure 4. Airport GPS reference receiver.

6 Gravity ties Apron reference gravity was tied to the IGSN71 values at SMD gravity points in or near Dodoma, Moshi, Tanga and Mtwara airports. The gravity ties were done with LaCoste&Romberg G 466 land gravimeter. Table3. Airport gravity values (P. Morgan personal comm.) Airport G value (IGSN71) Sigma Dodoma Moshi Tanga Mtwara Table 4. Derived apron gravity values Airport G value (IGSN71) Sigma Dodoma Kilimanjaro Tanga Mtwara Airborne gravity processing Free air gravity anomalies at aircraft level are obtained from Δg γ h 2 f f h δg δg g γ ( h N h N ) (1) z z0 eotvos tilt h γ 2 where f z is the gravimeter observation, f z0 the apron base reading, h the GPS vertical acceleration, g eotvos the Eötvös correction computed by the formulas of Harlan (1968), g 0 the apron gravity value,

7 - 7 - γ0 normal gravity, h the GPS ellipsoidal height and N the geoid undulation (EGM08 used throughout). γ 0 and the second order height correction in equation (1) is based on GRS80 definitions (Moritz, 1980). An altitude dependent atmospheric correction has been applied, see eg Hintze et al EGM08 values are derived from the pre computed geoid undulations grid Und_min2.5x2.5_egm2008_isw=82_WGS84_TideFree_SE provided by N. Pavlis, see The platform off level correction g tilt is based on a platform modeling approach as described in Olesen (2003). All data were filtered with a symmetric second order Butterworth filter with a half power point at 170 seconds, corresponding to a resolution of 6 km (half wavelength). The impulse response and spectral behavior of the filter are shown in Figure 5. See Olesen 2003 for further details regarding the airborne gravity data reduction. Apron base readings were performed each day before and after the flight in order to monitor drift of the airborne gravimeter and make a proper connection of airborne readings to the IGNS71 network. normalized weight transmission seconds 1E-3 1E-2 frequency, Hz Figure 5. Impulse response (normalized) and spectral representation of the filter

8 Data validation Weather conditions and thereby data quality has in general been good. We only encountered a few situations with more severe turbulence. The affected parts have been re flown. Figure 6 shows the acquired free air anomalies at flight altitude. The figure shows nice color agreement where lines intersect indicating consistent and healthy data. A closer examination of the misfit where lines intersect show a 3.2 mgal RMS error indicating 2.3 mgal average noise for the whole data set. This is very satisfying results and in line with other surveys flown under similar conditions. Figure 6. Free air anomalies at flight altitude

9 -9- Figure 7 shows the countrywide airborne gravity coverage. The plot shows that the June/July data blends seamlessly with the two other surveys, the 2012 and the September/October 2013 survey underscoring the good and even quality of the three surveys. This combined data set forms the basis for the computation of the Tanzanian geoid. The geoid computations are described in a separate re port. Figure 7. Gridded free air anomalies for the three surveys; the 2012 survey, the June/July 2013 survey and the September/October 2013 survey. Ph Dir Fax

10 Summary The survey was successfully completed and with very satisfying results. Cross over error estimates indicate a 2.3 mgal average noise on the data. The good data quality was achieved due to proper planning and preparations. The fact that we were able to fly at night contributed significantly to data quality. The successful completion was in large parts thanks to preparations and support from SMD. References Harlan, R. B.: Eotvos corrections for airborne gravimetry, J. Geophys. Res., 3, , Moritz H.: Geodetic Reference System Journal of Geodesy, 54, Olesen, A. V.: Improved airborne scalar gravimetry for regional gravity field mapping and geoid determination, Kort og Matrikelstyrelsen Technical Report no. 24, 55 pp, 2003

11 Appendix A: Flight summary MLHHSD-DTU flights Tanzania 2013 (5H-BEE) Date/JD* Flight from to Airborne Landed Airborne time 21 June/172 Test Dar-Dar Ferry Dar-Dod h37 25 June/176 F01 Dod-Dod h35 26 June/177 F02 Dod-Dod h34 27 June/178 F03 Dod-Dod h41 28 June/179 F04 Dod-Dod h03 29 June/180 F05 Dod-Dod h30 30 June/181 None 01 July/182 F06 Dod-Dod h55 02 July/183 F07 Dod-Kmj h21 03 July/184 F08 Kmj-Kmj h56 04 July/185 F09 Kmj-Kmj h47 05 July/186 F10 Kmj-Kmj h21 06 July/187 None 07 July/188 F11 Kmj-Kmj h55 08 July/189 F12 Kmj-Kmj h10 09 July/190 F14 Kmj-Kmj h35 10 July/191 F15 Kmj-Kmj h08 11 July/192 F16 Kmj-Kmj h58 12 July/193 F17 Kmj-Kmj h25 13 July/194 F18 Kmj-Kmj h41 14 July/195 None 15 July/196 F19 Kmj-Kmj h15 16 July/197 F20 Kmj-Zan h Ferry Zan-Tng h32 17 July/198 F21 Tng-Zan h Ferry Zan-Tng h35 18 July/199 F22 Tng-Dar h08 19 July/200 F23 Dar-Mtw h06 20 July/201 F25 Mtw-Mtw h07 22 July/203 F26 Mtw-Mtw h25 23 July/204 F27 Mtw-Mtw h11 24 July/205 F28 Mtw-Mtw h30 25 July/206 F29 Mtw-Mtw h47 26 July/207 F24 Mtw-Mtw h Ferry Mtw-Dar h32