| S.No. | Field / Checkpoint | Reference | Status |
|---|---|---|---|
| A. DGPS SETUP | |||
| A1 | Base + rover unit + L1/L2 frequency + GLONASS / GPS / Galileo Acceptance: Multi-constellation if available | Per device spec | OK NC NA |
| A2 | Base station coordinates (known + computed) Acceptance: Verified | Per CORS / project base | OK NC NA |
| A3 | RTK mode — single base / NTRIP / CORS Acceptance: Per setup | Per project | OK NC NA |
| B. OBSERVATION DATA | |||
| B1 | Point ID + time + satellites (≥ 5 typically) + HDOP Acceptance: Per surveyor | Quality threshold | OK NC NA |
| B2 | Coordinates (E, N, Z) + accuracy estimate (cm-level typical RTK) Acceptance: Per accuracy class | Per device | OK NC NA |
| B3 | Quality flag — fixed / float / DGPS / autonomous Acceptance: Accepted only if fixed | Fixed = best | OK NC NA |
| C. POST-PROCESSING | |||
| C1 | Data post-processed (if static observations) Acceptance: Per software | Per project standards | OK NC NA |
| C2 | Coords reconciled to project datum Acceptance: Per region datum | Per transformation parameters | OK NC NA |
DGPS (Differential GPS) and GNSS RTK (Real-Time Kinematic) are now standard tools for highway, township, large-area, mining, and infrastructure surveys. They provide centimetre-level accuracy (1-5 cm horizontal, 2-10 cm vertical) at 10-50× the speed of total station surveys for large areas.
But DGPS / GNSS data is only as good as: - Base station accuracy — if base is at wrong coordinates, all rover points are systematically off - Satellite geometry (HDOP / PDOP) - Multi-path effects (signals bouncing off buildings) - Ionospheric / tropospheric conditions - Datum + projection consistency - Fix vs Float vs Autonomous observation quality
The DGPS Observation Log captures every point's metadata: base reference, satellites visible, accuracy estimate, quality flag, datum used. Without it: - Disputed survey results can't be defended - Datum mismatches between survey + design cause systemic errors - Quality of individual points unknown — float vs fix not distinguishable - Post-processing impossible (need raw GPS data + observation conditions) - NHAI / MoRTH audit failures — surveyor competence challenged
Governed by IS 14252 (Setting Out) + Survey of India specifications + ISO 17123 series for instrument testing.
Standard DGPS / RTK observation workflow:
Step 1 — Base station setup: - Known-coordinate point (control point, CORS reference, or new point post-processed) - Base antenna on tripod over the point - Phase centre offset noted - Base coordinates entered (WGS-84 / project datum) - Transmission mode selected (UHF radio / GSM / internet NTRIP) - Verify base is broadcasting corrections
Step 2 — Rover setup: - Rover antenna + controller paired with base - Multi-constellation: GPS + GLONASS + Galileo + BeiDou (typically 4 systems = 15-25 satellites visible) - L1 / L2 / L5 frequencies enabled for ionospheric correction - Antenna height measured precisely - Rod / tripod plumb verified
Step 3 — Initialisation: - Wait for 'Fixed' status (full RTK accuracy) - Float = degraded, ~10-30 cm accuracy - Autonomous = single-receiver, ~5-15 m - HDOP < 2 = good geometry; > 4 = poor - Number of satellites: ≥ 5 (≥ 7 preferred)
Step 4 — Point capture: - Each point captured for 5-20 seconds (averaging multiple epochs) - Point ID, time, satellite count, HDOP, fix-type recorded - Coordinates: East, North, Height in selected datum - Accuracy estimate (1-sigma + 2-sigma) - Description / code (e.g., RT for road centre, TP for top of bank, KG for kerb edge)
Step 5 — Quality check: - Same-point repeatability — visit point twice, verify within 2 cm - Check on known point — survey a control point, compare to known coords - Loop closure — for traverse-mode use - Multi-path verification — points near buildings / dense canopy re-checked
Step 6 — Post-processing (for static observations): - Raw GNSS data downloaded to PC - Software: Leica Infinity / Trimble Business Center / Topcon Magnet / Open-source RTKLIB - Base + rover data processed together - Adjusted coordinates with ellipsoidal + orthometric heights - Datum transformation (WGS-84 to local grid via Indian Datum / Everest / UTM)
Common observation modes: - RTK (Real-Time Kinematic) — instant results, cm accuracy - Static (60+ minute occupation) — highest accuracy, mm-level, for control - Fast Static (10-30 min) — control points - Stop-and-Go — alternating moving + brief occupation - Continuous Kinematic — pavement / road profiles
Datum + projection (India-specific): - WGS-84 — global GPS reference - Indian Geodetic Datum (IGD-1995) — official Indian datum - UTM — Universal Transverse Mercator (Zones 42N, 43N, 44N, 45N, 46N for India) - Polyconic Projection — Survey of India historical - Local Grid — project-specific (often based on Lambert / Cassini) - MSL (Mean Sea Level) — orthometric heights; geoid model required from EGM2008 / Indian Geoid
1. Base station coordinates wrong — entered from outdated survey; all rover points systematically off by metres.
2. Datum mismatch — survey in WGS-84, design in local grid; conversion forgotten; survey data unusable.
3. Accepting Float observations — surveyor in hurry, accepts Float status (10-30 cm); building set out with that accuracy; out of tolerance.
4. Antenna height wrong — measured to base of rod, not phase centre; vertical error 5-15 cm.
5. Multi-path near buildings — RTK in urban canyon; signal bounces; coords shift; not flagged because shows 'Fixed'.
6. No same-point repeatability check — single observation accepted; later check shows error; can't trace back.
7. No check on known control point — survey done; no verification against project control; systematic error undetected.
8. HDOP / PDOP not logged — geometry poor at certain times of day; quality reduced; not captured.
9. Raw data not saved — only post-processed coords on file; later issue arises; can't re-process.
10. Ionospheric activity ignored — during solar storms, GPS accuracy degrades; survey done; data poor.
11. Geoid model not applied — height in ellipsoidal (h) used as orthometric (H); difference 30-90 m in India; gross error.
12. CORS subscription expired — NTRIP corrections stopped mid-survey; surveyor doesn't notice; observations in autonomous mode (5+ m error).
13. GLONASS / Galileo disabled — GPS-only mode; fewer satellites; poorer geometry; not enabled in old equipment.
14. Local datum transformation parameters wrong — using outdated transformation; project grid coords off by 1-3 m.
15. Crew not trained — DGPS used as 'press button to get coords'; quality concepts not understood; data accepted without scrutiny.
Companion formats: - Setting Out / Line Out Register (FMT-SIT-007) — DGPS feeds setting out - Traverse Register (FMT-SUR-002) — control surveys - Cross Section Format (FMT-SUR-005) — cross-sections from DGPS - Bench Mark Register — vertical control - 3A / 3D NHAI Notification Tracker (FMT-LAN-001) — DGPS surveys for ROW - Calibration Register (PMC-EQP-LOG-004) — DGPS calibration
Standards + references: - IS 14252:1995 — Setting Out of Buildings - IS 1500 (Survey of India) — Topographical surveying methods - Survey of India Manual of Geodetic Practices — official Indian geodetic procedures - ISO 17123 Parts 1-9 — Field procedures for testing geodetic + surveying instruments (Part 8 for GNSS) - IRC SP 19:2001 — Manual for Survey, Investigation + Preparation of Road Projects - NHAI Geodetic + Topographic Standards - NCC (National Centre for Cartography) — India CORS network reference - IGS (International GNSS Service) — global reference network - EGM2008 / Indian Geoid (NavIC + IRDS-1989) — geoid models for orthometric heights - Bureau of Indian Standards — Survey-related codes (IS 13201, IS 14252, IS 7204)