This article walks through the real job step by step, from the landowner's brief and the coordinate system to what was collected in the field and the files that were delivered, to show what data goes into a single topographic map. For the general principles, see What Is a Topographic Survey?, and for choosing equipment, GPS RTK vs Total Station vs Drone.
What was the brief for this job?
The brief was a topographic map that the architect and engineers could use to start design straight away. The landowner planned to develop a building on vacant land with large trees, an old concrete yard and some existing structures. The agreed scope was:
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| Item | Details |
|---|---|
| Area | Vacant land in central Bangkok, about 6,830 m² (4-1-7.5 rai), with road frontage on two sides |
| Site condition | Overgrown grass and shrubs, several large trees, an old concrete yard and small existing buildings |
| Purpose | Input for the next stage of engineering and architectural design |
| Coordinate system | UTM WGS 1984 Zone 47 · Geoid Model TGM2017 · levels referenced to mean sea level (MSL) |
| Items to survey | Existing ground levels, land boundary, roads, footpaths, drainage, utilities, structures and trees |
| Team | 1 survey engineer, 1 party chief and 5 survey assistants |
How were the benchmarks and coordinate system set up?
A benchmark (BM) is a permanent mark with known coordinates and elevation, used as the reference for surveying, design and construction. On this job the steps were:
- Install 2 permanent benchmarks on the kerb of the footpath next to the site, painted and numbered so they are easy to find.
- Fix the coordinates of the first benchmark with a GNSS receiver referenced to the network of continuously operating reference stations, then set the starting bearing from the first benchmark to the second.
- Process the satellite data for the main benchmark from about 1 hour of continuous observation, giving standard errors at millimetre level.
- Check the level on a reference benchmark (check level) before collecting data, to confirm the receiver has a full fixed solution and the heights are not drifting.
The resulting elevations were +0.2975 m MSL and +0.3665 m MSL. Values this close to zero are normal for the low-lying Chao Phraya river plain. The design team and contractor will use these two benchmarks to set floor and road levels.
What was collected in the field?
The field data fell into 4 groups. Every point has Northing, Easting and elevation.
- Existing ground levels on a grid of about 10 × 10 m, with extra points where needed, such as the edges of the concrete yard and breaks in level.
- Roads and footpaths, including the road level in front of the site, kerbs and walls, used to set building and access levels.
- Utilities: manholes, water valve covers, electricity poles and signs, both on the site and nearby.
- Trees larger than 25 cm in diameter, with species, such as rain trees, tamarind, coconut, palms, star gooseberry, jujube, papaya, devil trees (Alstonia) and bamboo clumps.
What did each instrument do on this job?
Urban work uses several instruments together, because each one has different strengths.
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| Instrument | Used for | Notes |
|---|---|---|
| GNSS receivers (RTK) | Fixing benchmark coordinates and recording levels in open areas | 4 sets, Real-Time Kinematic surveying |
| Total Station | Detail under trees, close to buildings, and traversing | Traverse accuracy 1 : 10,000 |
| Phantom 4 RTK drone + ground control points (GCP) | Aerial photos and the map overlay | Gives an overview of the site and its structures |
| AutoCAD Civil 3D | Computing and drafting the topographic map | Delivered as .dwg / .dxf |
The main instruments were calibrated before use, and the calibration certificates are attached in the report appendix.
How were boundary markers and wall corners used to define the boundary?
The site boundary on this job was defined from the boundary marker actually found on site together with the corners of the surrounding wall. The team found 1 boundary marker, but its number could not be read, so they also surveyed the wall corners around the site and computed an area of about 6,830 m².
The area is therefore an approximate figure for design, not a legal boundary. A boundary that matches the title deed needs a boundary survey by the Land Office. The difference between the two is explained in How a topographic survey differs from a land boundary survey.
What was delivered?
The landowner received 3 deliverables that the architect and engineers can use straight away.
- AutoCAD files (.dwg / .dxf) showing levels, boundary, roads, utilities, structures and trees.
- 2 A3 topographic map sheets: a line map and a map overlaid on the aerial photo, with a benchmark coordinate table.
- Survey report (PDF) with the survey method, benchmark and boundary marker coordinates, site photos, GNSS processing results and instrument calibration certificates.
Lessons from this job for surveying urban land
- Set benchmarks where they will not be removed. A footpath kerb or other permanent structure outside the construction area keeps the benchmark usable for the whole project.
- Bring more than one instrument. Tall buildings and large trees can block satellite signals; points where GNSS reception is poor are picked up with the Total Station.
- Survey beyond the boundary. Road levels, manholes and electricity poles around the site affect building levels, access and drainage.
- Boundary markers may be missing or unreadable. Tell the owner at the start that the surveyed area is a design figure, not a legal boundary.
- Record the species of large trees. This lets the architect decide early on whether to keep trees as part of the layout.
If the next stage needs soil investigation for foundation design, the borehole locations can be placed on the same topographic map. Read more in How many boreholes and how deep? or see SPN's topographic survey service.
The cost of a topographic survey depends on the size of the area, how overgrown it is, how much detail must be collected and whether a drone is used, so it is quoted per project. Request a free quote within 24 hours on the SPN contact page. Prices exclude travel and equipment mobilisation, which are charged by distance to site.
The figures and photos in this article come from a real SPN survey. The project name, landowner and site coordinates are not disclosed, and the map shown is a partial crop. The conclusions are explanatory and are not design advice for any other project.