GELE Surveying (Geomatics) — Route, Topographic and Modern SurveyingRevision Notes

Formulas

Exam Tips

• When a board exam problem says 'the ground rises through N contour lines,' subtract 1 to get N-1 intervals.
• If the problem gives you a slope and asks for horizontal distance, rearrange: L = ΔH / slope.
• Sketch a quick profile from contour data to avoid errors in reading V-directions.
• Index contours are labeled — use them to verify your elevation count.

Key Points

• A topographic survey captures the three-dimensional shape of the ground surface, recording planimetric features (roads, structures) AND elevations.
• Contour lines are imaginary lines on a map connecting points of equal elevation. The vertical distance between successive contours is the contour interval (CI).
• Contour interval selection depends on scale and terrain: small CI for gentle terrain, larger CI for steep terrain.
• Rule 1 — Equal spacing: CI is constant throughout one map sheet.
• Rule 2 — Steep vs. gentle: closely spaced contours = steep slope; widely spaced = gentle slope.
• Rule 3 — No crossing: Contours never intersect except at a vertical cliff or overhang.
• Rule 4 — Closure: Every contour line closes on itself, either within the map sheet or beyond its borders.
• Rule 5 — Valley rule: In a valley (stream channel), contour V-shapes point UPSTREAM (uphill).
• Rule 6 — Ridge rule: On a ridge (divide), contour V-shapes point DOWNHILL.
• Rule 7 — Every 5th contour (index contour) is drawn heavier for readability.
• Spot elevations and benchmarks supplement contours for summits, saddles, and critical points.
• Slope (%) = (elevation difference / horizontal distance) × 100
• Number of contour intervals between two contours = number of lines – 1 (count INTERVALS, not lines).

Definitions

Section Title

Topographic Surveying and Contour Lines

Common Mistakes

• Counting LINES instead of INTERVALS — 6 contour lines = 5 intervals. Always compute ΔH = n_intervals × CI.
• Confusing valley V-direction: V points UPSTREAM (uphill) in valleys. Memory aid: 'V = into the Valley, pointing uphill like water flows.'
• Assuming contour spacing is proportional to actual spacing on the ground — always convert using map scale first.
• Mixing up steep and gentle slope visual cues — closely spaced = STEEP, widely spaced = GENTLE.

Formulas

Exam Tips

• Memorize the two-step process: (1) compute S = H/f, (2) apply D = d × S or d = D/S.
• When the problem gives altitude above sea level and ground elevation, always subtract to get H.
• For area problems, use S² — do NOT forget the square.
• If asked for the flying height for a desired scale: H = S × f. Quick check: larger scale number → higher aircraft.
• Board problems often mix mm and m — convert everything to meters at the start.

Key Points

• Photogrammetry is the science of obtaining reliable measurements from photographs, primarily vertical aerial photos.
• A VERTICAL aerial photo is taken with the camera axis truly vertical (no tilt).
• Photo scale = f / H, where f is the camera focal length and H is the flying height ABOVE the terrain (not above sea level).
• If the terrain has variable elevation, the effective flying height H changes across the photo, meaning scale varies across the photo.
• Ground distance = photo distance × scale denominator (1/scale).
• Photo distance = ground distance / scale denominator.
• Scale is a dimensionless ratio — ensure f and H are in the SAME units before dividing.
• Common focal lengths in Philippine board problems: 150 mm, 152 mm (6-inch), 210 mm.
• Scale expressed as 1:S means S = H/f.
• Relief displacement: objects taller than the datum appear displaced outward from the principal point on a vertical photo.
• Stereoscopic pairs (overlapping photos ~60%) allow 3D measurement and the basis of digital elevation models (DEMs).
• Photogrammetry reduces fieldwork significantly — critical for large-scale mapping of remote Philippine terrain (mountains, islands).

Definitions

Section Title

Photogrammetry and Aerial Photo Scale

Common Mistakes

• Using H above sea level instead of H above terrain. If terrain elevation = 200 m AMSL and aircraft altitude = 2200 m AMSL, then H = 2200 – 200 = 2000 m.
• Not converting focal length from mm to meters before computing scale: f = 150 mm = 0.150 m.
• Confusing photo scale with map scale — they obey the same formula but photo scale varies with terrain relief.
• Forgetting to square the scale denominator when converting AREAS: Area_ground = Area_photo × S².
• Taking reciprocal incorrectly: Scale = 1:10,000 means 1 unit on photo = 10,000 units on ground, NOT the reverse.

Formulas

Exam Tips

• Echo sounder problems almost always test the depth = vt/2 formula — remember to divide by 2.
• If the problem gives altitude of aircraft above the sea surface for a coastal survey, check whether it is a photogrammetry or hydrography question.
• Hydrographic survey questions in the PRC board often appear as application/conceptual questions rather than computation.

Key Points

• Hydrographic surveying maps the underwater features of water bodies: depths (soundings), shorelines, underwater topography, and current/tidal data.
• Applications in the Philippines: harbor design, dredging (ports like Manila, Cebu, Davao), bridge foundation investigation, flood control, and irrigation.
• Horizontal positioning: traditionally by intersecting angles from shore stations; now by GNSS (GPS) onboard the survey vessel.
• Depth measurement: SOUNDING. Methods: (a) lead line (manual, accurate for shallow water), (b) echo sounder / SONAR (standard for most surveys).
• Echo sounder principle: depth = (velocity of sound in water × travel time) / 2. Typical sound velocity in seawater ≈ 1500 m/s.
• Datum for depth: tidal datum (e.g., Mean Lower Low Water, MLLW) — the reference for nautical charts. Not the same as land survey datum.
• Tidal corrections must be applied to reduce soundings to the tidal datum.
• Sounding lines (track lines) are run at regular intervals; depth values are plotted to produce bathymetric (underwater contour) maps.
• Isobaths are contour lines of equal water depth on a bathymetric map.
• Sedimentation studies and dredge volume calculations use successive bathymetric surveys.

Definitions

Section Title

Hydrographic Surveying

Common Mistakes

• Using one-way travel time instead of two-way: depth = v × t / 2, not v × t.
• Confusing tidal datum with the national land datum (e.g., NAMRIA mean sea level datum).
• Forgetting that sound velocity varies with water temperature and salinity — problems will specify the value to use.

Formulas

Exam Tips

• Board exam MCQs on GNSS typically test: minimum satellites (4), accuracy levels (autonomous vs. DGPS vs. RTK), and coordinate reference systems.
• Total station specs (e.g., '2 arc-second instrument, ±2mm+2ppm EDM') may appear in precision traverse problems.
• GIS questions focus on conceptual understanding: layers, attributes, spatial analysis, and topology.
• RA 544 questions are common in the Professional Practice portion — memorize the distinction between CE and GE scope of work.

Key Points

• GPS (Global Positioning System, USA) is one constellation of the broader GNSS (Global Navigation Satellite System), which includes GLONASS (Russia), Galileo (EU), BeiDou (China), and QZSS (Japan, with coverage over the Philippines).
• GNSS determines 3D position by measuring pseudoranges from ≥ 4 satellites simultaneously; minimum 4 satellites for x, y, z, and clock correction.
• Standalone (autonomous) GPS: ±5–10 m accuracy — sufficient for GIS data collection but NOT for engineering control.
• Differential GPS (DGPS): uses a base station at a known point to transmit corrections → sub-meter accuracy.
• RTK (Real-Time Kinematic) GNSS: phase measurements + radio link from base station → 1–2 cm accuracy in real time. Standard for control surveys and stakeout in the Philippines.
• Static GNSS: long occupation times (20–120 min), post-processed → mm-level accuracy for geodetic control.
• Total Station: integrates (a) EDM (Electronic Distance Measurement), (b) electronic theodolite, and (c) onboard microprocessor. Measures horizontal/vertical angles + slope distance, computes coordinates directly.
• Total station is the standard field instrument for traverse, detail survey, stakeout, and as-built surveys.
• Robotic (Motorized) Total Station: operator works alone; instrument tracks the prism automatically.
• 3D Laser Scanner (LiDAR): measures millions of points per second by pulsed laser; produces point clouds for BIM, heritage documentation, and infrastructure surveys.
• GIS (Geographic Information System): software platform for storing, querying, analyzing, and mapping geospatial data. Survey results are the primary input.
• RA 544 (Revised Surveyor's Law, Philippines): requires all land and cadastral surveys to be done by licensed Geodetic Engineers (GEs). Civil Engineers use surveying tools on construction projects but cadastral surveys require GE licensure.
• NAMRIA (National Mapping and Resource Information Authority): the Philippines' national mapping agency; sets geodetic datums and provides control monuments.

Definitions

Section Title

Modern Surveying: GPS/GNSS, Total Stations, and GIS

Common Mistakes

• Confusing GPS (one constellation) with GNSS (all constellations) — use GNSS as the general term in modern practice.
• Stating that RTK accuracy is ±5 m — RTK gives 1–2 cm. Autonomous GPS gives ±5–10 m.
• Assuming total stations use optical chains only — EDM is electronic; the prism (reflector) is required for standard EDM, but reflectorless models exist.
• Confusing GIS with GPS — GIS is spatial data management software; GPS is a positioning technology. They are complementary, not synonymous.
• Overlooking RA 544: Civil Engineers cannot sign cadastral and land surveys — only licensed Geodetic Engineers can.