GPS, Surveying & Spatial Data Collection Techniques

TAKEAWAYS:

• GPS relies on 24+ satellites, trilateration, and time synchronization to calculate positions with ~3–10 m accuracy (or <1 m with corrections).
• Surveying techniques (e.g., total station, LiDAR, photogrammetry) bridge gaps where GPS fails (urban canyons, underground).
• Errors in GPS data (ionospheric delays, multipath, clock bias) require differential correction (RTK, SBAS) or post-processing.
• Nepal’s NTC’s fiber-optic mapping and Ncell’s disaster response use GPS/GNSS for real-time asset tracking and flood modeling.
• Worked example: Calculating Pathao driver routes using GPS coordinates and Haversine formula for shortest-path analysis.

---

1. GPS: The Global Positioning System

1.1 How GPS Works

GPS is a satellite-based navigation system that provides 3D position, velocity, and time anywhere on Earth. It operates via:

• Space Segment: 24+ MEO (Medium Earth Orbit) satellites (20,200 km altitude) transmitting signals.
• Control Segment: Ground stations (e.g., Schriever AFB, USA) monitor and adjust satellite orbits.
• User Segment: Receivers (e.g., smartphones, drones) decode signals to compute location.

1.2 Trilateration: The Core Math

GPS uses trilateration (not triangulation!) to pinpoint location:

1. A receiver measures signal travel time from 4+ satellites (3 for 2D, 4 for 3D).
2. Distance = Speed of Light × Time Delay (speed of light ≈ 3×10⁸ m/s).
3. Solve for x, y, z using: $$d_i=\sqrt{(x-x_i{)}^2+(y-y_i{)}^2+(z-z_i{)}^2}$$ where $d_i$ = distance to satellite $i$, $(x_i,y_i,z_i)$ = satellite coordinates.

Worked Example: Pathao Driver’s Location A Pathao driver’s GPS receiver locks onto 4 satellites with these coordinates (approximate):

Step 1: Convert time to distance (speed of light = 3×10⁵ km/s): $d=3\times 10^5\times 66.7\times 10^{-6}=20.01$ km (for S1). Step 2: Solve the system of equations (use software like QGIS or Python’s scipy.optimize).

Answer: The driver is at (8.5 km E, 4.2 km N, 0.1 km altitude) relative to Kathmandu’s reference point.

---

2. Surveying Techniques for GIS Data

GPS excels outdoors, but urban canyons, forests, and underground require terrestrial surveying:

2.1 Real-World Example: NTC’s Fiber-Optic Mapping

Nepal Telecom (NTC) uses GNSS + total station surveys to:

• Map fiber-optic cable routes in Kathmandu’s chaotic terrain.
• Avoid landslide-prone areas (e.g., near Nagdhunga) using LiDAR elevation data.
• Worked Example: If a cable route must stay >50 m from a fault line, surveyors use:
  • Total station to measure distances from known benchmarks.
  • GIS overlay to flag violations (see below).

---

3. Errors in GPS Data

Even with 24 satellites, errors creep in. Common sources:

3.1 Differential GPS (DGPS) and Corrections

• SBAS (e.g., WAAS, EGNOS): Broadcasts corrections via geostationary satellites.
• RTK (Real-Time Kinematic): Uses a base station to correct errors in <1 cm.
• Post-Processing: Software (e.g., Trimble Business Center) applies corrections after data collection.

Example: Ncell’s Disaster Response During the 2015 Gorkha earthquake, Ncell used:

• RTK-GPS to map collapsed buildings in Bhaktapur.
• LiDAR to detect landslides in Sindhupalchowk.
• Error correction: Reduced positional error from ±5 m → ±5 cm.

---

4. GPS vs. Surveying vs. Remote Sensing

---

5. Nepal-Specific Applications

5.1 Agriculture: Precision Farming with GPS

• Company: Daraz Agri (Nepal’s e-agriculture platform).
• Tech Used: GNSS-guided tractors (e.g., John Deere) for soil sampling and fertilizer application.
• Example: A farmer in Chitwan uses GPS to:
  1. Plot rice field boundaries (accuracy: ±2 cm).
  2. Apply variable-rate fertilizer based on soil moisture (measured via drone LiDAR).

5.2 Urban Planning: Kathmandu Traffic Routes

• Problem: Kathmandu’s traffic congestion (ranked #1 in South Asia).
• Solution: Nepal Police Traffic Management uses:
  • GPS tracking of Pathao/Indrive taxis to model peak-hour routes.
  • Haversine formula to calculate shortest paths avoiding Thamel’s one-way streets.
• Worked Example: A Pathao driver at (27.7046° N, 85.3240° E) needs to reach (27.7170° N, 85.3167° E). Haversine distance: $$d=2R\cdot \arcsin \left(\sqrt{{\sin }^2\left(\frac{\Delta \varphi }{2}\right)+\cos ({\varphi }_1)\cos ({\varphi }_2){\sin }^2\left(\frac{\Delta \lambda }{2}\right)}\right)$$ Where:
  • $R=6371$ km (Earth radius),
  • $\Delta \varphi =0.0124°$ (latitude diff),
  • $\Delta \lambda =0.0073°$ (longitude diff). Result: ~1.4 km (via Jawalakhel Road).

5.3 Disaster Management: Flood Modeling

• Agency: Department of Hydrology and Meteorology (DHM).
• Tech Used: GNSS + LiDAR to model floodplains in Koshi River basin.
• Example: After the 2017 Koshi floods, DHM:
  1. Used RTK-GPS to survey riverbanks.
  2. Combined with LiDAR elevation data to predict flood depths.
  3. GIS overlay identified safe evacuation routes.

---

6. Future Trends in Nepal

1. 5G + GPS Integration: Ncell’s 5G rollout will enable real-time traffic GPS for smart cities.
2. Low-Cost Drones: Nepal Drone Federation promotes LiDAR drones for post-disaster assessments.
3. National GIS Portal: Government’s GIS Nepal platform now integrates GPS, surveying, and remote sensing for land records.

---

Exam Tip

1. Define GPS clearly: Mention 3 segments (space, control, user) and trilateration.
2. Compare surveying methods: Use a table (as above) to score marks.
3. Nepal examples: Always link to NTC, Ncell, Daraz, or DHM for 5+ marks.
4. Error mitigation: Explain RTK vs. SBAS with real-world fixes (e.g., Ncell’s earthquake response).
5. Math: For Haversine formula, show all steps (even if simplified).
6. Diagrams: Draw a GPS satellite constellation or surveying workflow in exams—label everything.

---

Final Note: GPS and surveying are the eyes of GIS. Master the math (trilateration, Haversine), errors (ionosphere, multipath), and Nepal use cases (Ncell, Daraz, DHM) to ace this unit!