NATURE OF PROJECT: TOPOGHRAPHY & PHOTOGRAMMETRY SURVEY
CLIENT: PETRONAS GAS BERHAD
LOCATION: LONG BEDIAN, SARAWAK
INTRODUCTION
Flood mitigation study for Block Valve 113, PETRONAS Gas Berhad, Sabah-Sarawak Gas Pipeline had been conducted prior to the flooding incident occurred on February and December of 2017. The flood event had damaged the sensitive electrical equipment at BV113.
As for the design process, it requires detail topography information of the survey area. To study the nature of the flooding incident occurred at the BV113 station, the surrounding area of the block valve station must be taken into consideration. The aerial survey was conducted to determine the updated terrain model of the catchment area which is 88km2.
In this project, the usage of advanced and modern geospatial technologies, specifically topographic laser scanning or also known as light detection and ranging (LiDAR) for mapping landslide inventory, evaluating LiDAR-derived landslide causal factor maps, analysing landslide susceptibility, assessing landslide hazard and associated risk in Kota Kinabalu and Kundasang, Sabah.
TOPOGRAPHY SURVEY
PETRONAS Gas pipeline system runs 512km from Kimanis, Sabah to Bintulu, Sarawak. Block Valve stations were installed at every 20km interval along the Sabah-Sarawak gas pipeline (SSGP). The project location is located at BV113 (KP298+200).
Figure 1: Area of topographic survey
Before establishing survey control points, existing PETRONAS Benchmark needed to be verified beforehand. PETRONAS permanent benchmarks that were used for the project are PB1294A and PB1294B. Their location are approximately 300 meters apart and visible to one another. The method to verify the permanent benchmark are Radio Real Time Kinematic (RTK Radio). This method can deliver realtime positioning at the field (Sholarin & Awange, 2015).
Survey control points were established in two location at BV113. Due to the distance of the verified benchmark are 4 kilometers away, fast-static survey method was adopted and only 1 control point is required for this method of observation (Kilford, 1979). The control point known as Base is at the PB1294B and the Rover are the PB1298A and PB1298B. Fast-static is a post processing survey method, hence the data were collected and processed using the GNSS receiver’s software. The requirement time of observation for Rover GNSS receivers for fast-static survey are 30 minutes (Kilford, 1979).
Figure 2: Location of the Control Points
Topographic Survey is a survey that gathers data about the elevation of points on a piece of land and can be presented as a 3-dimensional model. All topographic features and the ground terrain of a piece of land were mapped relative to the project coordinate reference system and vertical datum. Radio RTK were used by using PB1294A as the base station and 2 GNSS receivers as a rover. The ground level at BV113 were collected with 1m x 1m grid spacing from KP298+200 to KP298+400. Other man-made features such as wooden shack, gabion and concrete drain were also recorded using the receivers. These features are then to use as a GCP for the low altitude UAV survey.
Figure 3: Layout of the longsection and crossection
The route to the Sg Apoh River crossing is covered by tall grass and Low flying altitude was carried out during the data collection process with 75 meters flying height above the ground level with 85% front overlap and 70 % side lap. The advantage of low flying altitude is in higher resolution (Anderson & Gaston, 2013). The 258 aerial photos were processed to generate the orthophoto and Digital Surface Model (DSM) of the river crossing.
Figure 4: Digital surface model of low flying altitude UAV
Figure 5: 3Dimensional of the point cloud and pipeline
PHOTOGRAMMETRY SURVEY
The ground control points work is carried out using 2m x 2m survey markers and GNSS receivers. The method of observation is fast static with the base station is located at Long Bedian coffee shop and the rover were placed at the survey markers as GCP. The coordinate system used for the project are in CM114E, which is a Transverse Mercator projection coordinate system with the specific central meridian value given by PETRONAS and the JUPEM geoid is used for the height reference. A total of 15 GCP were observed at the field.
Figure 6: Photogrammetry block Survey planning
UAV drone used for this project are J-Hawk V2 with the capability of flying autonomous and the range can go up to 10km from home. J-Hawk is considered a suitable drone for mapping large area rather than a quadcopter drone. Equipped with a 15-megapixel camera and light body, the drone can go airborne for more than 90 minutes.
Although the survey area is 88km2 as requested, regularly more data is captured than the proposed value. Data captured are in RAW format and the flying altitude varies because of the undulating terrain in the survey area. Altitude from 350 meters to 600 meters above ground level are used in the data collection work.
Figure 7: Camera Location
Based on the Figure 7, the numbers from 1 to 9 refers to the number of overlapping images of aerial photos. The configuration in the mission planner is 80% frontlap and 60% sidelap. Some photo even has higher than 9 overlap and some cases have only one overlap due to the photo taken during the aircraft turning to the next flight line.
Processing stages are divided into 4 phases consisting of align aerial photo, build dense cloud, orthophoto and filtering.
Figure 8: DTM and DSM of the catchment
