Accuracy Analysis of GNSS-based Public Surveying and Proposal for Work Processes

GNSS관측 공공측량 정확도 분석 및 업무프로세스 제안

Abstract

Currently, the regulation and rules for public surveying and the UCPs (Unified Control Points) adapts those of the triangulated traverse surveying. In addition, such regulations do not take account of the unique characteristics of GNSS (Global Navigation Satellite System) surveying, thus there are difficulties in field work and data processing afterwards. A detailed procesure of GNSS processing has not yet been described either, and the verification of accuracy does not follow the generic standards. In order to propose an appropriate procedure for field surveys, we processed a short session (30 minutes) based on the scenarios similar to actual situations. The reference network in Seoul was used to process the same data span for 3 days. The temporal variation during the day was evaluated as well. We analyzed the accuracy of the estimated coordinates depending on the parameterization of tropospheric delay, which was compared with the 24-hr static processing results. Estimating the tropospheric delay is advantageous for the accuracy and stability of the coordinates, resulting in about 5 mm and 10 mm of RMSE (Root Mean Squared Error) for horizontal and vertical components, respectively. Based on the test results, we propose a procedure to estimate the daily solution and then combine them to estimate the final solution by applying the minimum constraints (no-net-translation condition). It is necessary to develop a web-based processing system using a high-end softwares. Additionally, it is also required to standardize the ID of the public control points and the UCPs for the automatic GNSS processing.

공공측량/통합기준점측량 작업규정은 기존 트래버스 측량 작업규정을 준용하고 있으며, GNSS관측 특성을 정확하게 반영하지 않아서 현장 작업과 자료처리에 어려움이 있다. 또한, GNSS관측 자료처리 절차에 대한 규정이 명확하지 않고, 정확도 검증방법 역시 일반적인 기준과 차이가 있다. 본 연구에서는 현재 규정을 분석하고 적절한 업무프로세스를 제안하기 위해 공공기준점 측량과 유사한 시나라오를 바탕으로 짧은 세션(30분) 데이터를 처리했다. 서울특별시 네트워크 RTK (Real Time Kinematic) 기준점에 대해서 3일간 동일한 시간대 결과를 비교했으며, 하루 중 시간에 따른 결과를 비교해서 전반적인 자료처리 정확도를 평가했다. 대류권 지연오차 추정여부에 따른 정확도 차이를 동시에 분석했으며, 추정결과는 24시간 정지측량 결과와 비교했다. 대류층 지연오차를 추정하는 것이 정확도와 좌표안정성 향상에 유리하며, 평균제곱근오차는 대략 평면 5mm, 수직 1cm 수준으로 추정되었다. 본 연구결과를 바탕으로 통합기준점을 포함한 동시관측 일간해를 추정하고, 이를 통합하여 최소제약조건을 통해 최종해를 결정하는 업무프로세스를 제안한다. 이를 위해서는 학술용 자료처리시스템을 이용한 자료처리자동화시스템이 구축되어야하며, GNSS자료처리를 위해 통합기준점과 공공기준점 코드를 표준화해야 한다.

Fig. 1. Configuration of GNSS stations used in the study

Fig. 2. Horizontal error of each session (Case 1) where each symbol represents one session (30 minutes). The tropospheric delay parameters were not estimated in Case 1. The time span of each solution is defined as follows: Session O (19:00-19:30), P (19:30-20:00), Q (20:00-20:30), R (20:30-21:00) of each day

Fig. 3. Horizontal error of YONS without estimation of tropospheric delay (Case 2) on 2018-06-28 (DOY 179). Each symbol represents one session (30 minutes)

Fig. 4. Horizontal error of YONS with estimation of tropospheric delay (Case 3) on 2018-06-28 (DOY 179). Each symbol represents one session (30 minutes)

Fig. 5. Vertical error estimated with (Case 3) and without (Case 2) tropospheric delay. The 1-hr interval rainfall was plotted as a bar with numbers in unit of centimeter

Fig. 6. Results of the suggested strategy for the public surveying (Case 4)

Fig. 7. Suggested strategy for public surveying based on GNSS observation

Table 1. GNSS-based public surveying of triangulation control points (NGII, 2015a)

Table 2. Tolerance of field check

Table 3. Criteria for network adjustment with one known point

Table 4. Summary of test cases used in this study (Year 2018) where YONS is used for the estimation. No tropospheric delay was estimated for SUWN due to the correlation between stations. All times are given in GPST

Table 5. Reference coordinates of the stations for comparison. All coordinates are referenced to the IGS14 reference frame (Reference epoch: 2018-06-27 12:00:00) by constraining to the coordinates of IGS station SUWN

Table 6. GNSS processing models used in the study

Table 7. Statistics of Cases 2 and 3

Table 8. Observation chart of Case 4. The time span of each solution is defined as follows: Session O (19:00-19:30), P (19:30-20:00), Q (20:00-20:30), R (20:30-21:00). YO[L/M/N][O/P/Q/R/] are actually the same station, that is, YONS, but treated as different stations for stability comparison of the solutions. The symbol ● represents that there is an observation at the specified session. SUWN was used as a reference station to be aligned to IGS14 reference frame and to reduce the correlation of the tropospheric parameters between stations

Table 9. Statistics of Cases 1 and 4