NivaarExam PrepOfficial exam papers ↗

18-Geom-B3 Networks and Precise Engineering Surveys · December 2018

Question 4 of 12: Deformation Monitoring by GNSS

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format: Closed-book, 3 hours, calculator permitted. TEN questions constitute a complete paper — Part A: all of #1–#8; Part B: one of #9/#10; Part C: one of #11/#12. All twelve questions are solved here for completeness. Most answers are essay-format; Q5, Q6 and Q9 carry short verified numeric illustrations.

Reference texts: Wolf, Ghilani & De Blij, Elementary Surveying: An Introduction to Geomatics (15th ed., Pearson); Mikhail & Gracie, Analysis and Adjustment of Survey Measurements (Van Nostrand, 1981); Kavanagh & Slattery, Surveying with Construction Applications; Hofmann-Wellenhof, Lichtenegger & Wasle, GNSS (Springer, 2008); Kahmen & Faig, Surveying (de Gruyter); Chrzanowski et al. on deformation analysis; USACE Structural Deformation Surveying (EM 1110-2-1009); ISO 17123 field-test procedures. Canadian frame throughout (NAD83(CSRS), CGVD2013).

Question 4: Deformation Monitoring by GNSS (8 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

GNSS deformation monitoring determines the movement of a structure or the ground by tracking, over time, the three-dimensional coordinates of GNSS antennas fixed to the object relative to stable reference antennas. Its defining characteristics are:

Relative (differential) carrier-phase positioning. Monitoring points and one or more reference points on stable ground carry GNSS receivers observing simultaneously. Short baselines are processed by double-differenced carrier phase, which cancels the satellite and receiver clocks and largely removes correlated atmospheric errors, delivering millimetre-level relative coordinates. Only the relative displacement matters, so common-mode errors between object and reference cancel.

Continuous, automatic, all-weather operation. Unlike optical methods, GNSS needs no line of sight between points, works day and night in any weather, and can run unattended 24/7 with data streamed to a processing centre. This makes it ideal for continuously monitoring dams, bridges, large structures, slopes, subsidence areas and tectonic motion (structural health monitoring / early-warning systems).

Two operating modes. Campaign (epoch) mode re-occupies the points periodically (e.g. yearly) to detect slow, long-term movement; continuous mode (CGPS) records permanently, resolving both slow trends and dynamic response (e.g. bridge oscillation) when high-rate data are used.

Strengths and limitations. Strengths: 3-D absolute-datum results (in NAD83(CSRS)/ITRF), automation, no inter-point visibility, homogeneous precision. Limitations: vertical precision is typically 2–3× worse than horizontal; requires a clear sky view (fails under canopy, in tunnels or against tall walls); multipath near reflective structures; and each point needs its own receiver, so dense spatial coverage is costly — hence GNSS is usually combined with total stations, levelling, InSAR or geotechnical sensors in an integrated scheme.