SinkAlert · Sub-project

Drone Railway Topographic Mapping

UAV-based topographic survey & map production for the Eastern railway corridor

NEW SUB-PROJECTAPPROACH · v0.1

🗺️ Map of the railway line — open the map (Eastern line drawn over Thailand), and download the line for the drone company: ⬇ GeoJSON · ⬇ KML

Objective. Construct a survey-grade topographic map along a 310.38 km railway corridor (Eastern line, สายตะวันออก) using unmanned aerial vehicles (UAVs / drones), in place of — or alongside — traditional ground survey. This sub-project defines the technical approach, sensor selection, positioning strategy, accuracy targets, and deliverable set for the corridor survey.

1 · TOR context (งานสำรวจทางรถไฟ)

The requirement sits under section 4.2.1 — งานสำรวจทางรถไฟ ("railway survey work"), which calls for:

Railway survey (สำรวจทางรถไฟ) across the Eastern line's operational area.
Topographic survey (การสำรวจภูมิประเทศ / Topographic Survey) — capturing terrain detail along the corridor.
Topographic map (Topographic Map) — producing a map showing terrain detail (แผนที่แสดงรายละเอียดภูมิประเทศ).

⚠️ Scope to confirm. The corridor length (310.38 km) and Eastern-line designation are read from the provided TOR excerpts. Exact station limits, required map scale, contour interval, and corridor width are not yet extracted — these must be confirmed against the full TOR before a priced bid. No figures below are assumed beyond what is stated here.

2 · Why drones for railway topography

Speed

Corridor capture in days–weeks versus months of conventional ground survey along an active rail line.

Safety

No survey crews on or beside live track — the aircraft works from a standoff, keeping people off the right-of-way.

Coverage

Long, narrow corridors map efficiently with fixed-wing VTOL or multi-rotor aircraft on programmed flight lines.

Accuracy

RTK/PPK positioning delivers survey-grade centimetre-level results suitable for topographic mapping.

3 · Sensor choice — photogrammetry vs LiDAR

Method	How it works	Best for	Accuracy (citable)
Photogrammetry (RGB)	Overlapping photos → Structure-from-Motion (SfM) reconstructs the surface	Open, cleared ground; full-colour orthomosaic + visual context	1–3 cm horizontal · 2–5 cm vertical (with RTK/GCP)
LiDAR (laser)	Laser returns → classified point cloud, penetrates vegetation to bare earth	Vegetated / low-light sections; bare-earth DTM under canopy	2–5 cm
Hybrid	Both sensors in one flight	Full corridor: imagery + reliable ground even where vegetation intrudes	Combined above

For a rail corridor — largely open and cleared — photogrammetry is the primary method, with LiDAR added on vegetated or canopy-covered segments. A hybrid payload gives the most robust result across the full 310.38 km.

4 · Positioning — RTK / PPK / GCP

Survey-grade absolute accuracy depends on positioning, not just the camera:

RTK (Real-Time Kinematic) — live corrections during flight; 2–5 cm when the correction link is strong.
PPK (Post-Processed Kinematic) — corrections applied after flight; consistently <3 cm, more robust where the link drops.
GCP (Ground Control Points) — surveyed ground markers to tie the model to real coordinates; reduce or remove GCPs when RTK/PPK is used.
Hybrid — RTK in flight + PPK validation after = maximum reliability.

Survey-grade receivers (e.g. Septentrio-based) advertise 0.6 cm + 0.5 ppm RTK horizontal accuracy. Drone photogrammetry with RTK/GCP typically lands at 1–3 cm horizontal / 2–5 cm vertical. These are manufacturer/published figures, not this project's measured results — our measured accuracy is reported after the pilot, not before.

5 · Workflow

Corridor flight planning — programmed flight lines along the 310.38 km track with sufficient overlap (front & side lap) and target GSD (≈5 cm survey-grade; sub-cm achievable with high-end payloads).
Data capture — overlapping imagery (and/or LiDAR scans) collected segment-by-segment along the corridor.
Processing — SfM/MVS photogrammetry (Pix4D / Metashape class) → dense point cloud → georeferenced output.
Elevation models — DSM (surface, incl. structures/vegetation) → DTM (bare earth).
Topographic map — contours + breaklines generated from the DTM, delivered as the map product.

6 · Deliverables

Orthomosaic

Georeferenced, seamless colour image of the corridor.

DSM / DTM

Digital Surface Model + bare-earth Digital Terrain Model.

Contour lines

Topographic contours generated from the DTM (interval per TOR).

Topographic map

The primary TOR deliverable — terrain detail map of the rail corridor.

7 · Bid scope (proposed — confirm vs TOR)

Item	Value
Corridor	Eastern railway line (สายตะวันออก)
Length	310.38 km (from TOR excerpt)
Method	UAV photogrammetry (primary) + LiDAR (vegetated segments)
Positioning	RTK + PPK + GCP, survey-grade
Timeline & cost	PROPOSED — pending corridor width, contour interval, map scale, and station limits from the full TOR