GELE Photogrammetry & Cartography — Stereoscopy, DEM and OrthophotoRevision Notes
Revision notes for GELE Photogrammetry & Cartography — Stereoscopy, DEM and Orthophoto. Short, focused, and designed for the week before exam day. Use these when you are already familiar with the chapter and need a quick refresh on the high-yield items Professional Regulation Commission (PRC) — Board of Geodetic Engineering tests.
Exam context
On the GELE 2026, the Photogrammetry & Cartography subtest carries a "Core" weight in Professional Regulation Commission (PRC) — Board of Geodetic Engineering's pattern. Stereoscopy, DEM and Orthophoto lands at position 3rd out of 6 in the standard review order. Target score is 70% weighted average, no sub-test below 50%, and roughly a meaningful share of items come from Photogrammetry & Cartography on a typical GELE paper.
Stereoscopy, DEM and Orthophoto - Revision Notes
This chapter covers three interconnected photogrammetric concepts essential for the PRC Geodetic Engineer Licensure Examination: (1) stereoscopy — the extraction of three-dimensional information from overlapping aerial photographs; (2) Digital Elevation Models (DEM) — gridded or triangulated representations of terrain elevation; and (3) orthophotos — geometrically corrected imagery with uniform, map-measurable scale. Mastery of these topics is critical because they underpin all modern photogrammetric mapping workflows, including those used in the production of Philippine base maps by NAMRIA under the authority of RA 4374/RA 8560 and PD 1529.
Sections
Formulas
Example
Air-base B = 900 m, flying height H = 1500 m → B/H = 900/1500 = 0.60
Formula
B/H = air-base / flying height
Variables
B = air-base (distance between successive exposure stations, m); H = flying height above mean terrain (m)
Application
Evaluates the geometric strength of height determination. A typical B/H ≈ 0.60 for 60% overlap. Larger B/H → stronger heights but more relief displacement.
Example
60% overlap, 230 mm format at scale 1:10 000 → GSD_strip = 0.230 × 10 000 = 2300 m; B = (1 − 0.60) × 2300 = 920 m
Formula
air-base B = (1 - p/100) × GSD_strip
Variables
p = forward overlap percentage (%); GSD_strip = ground dimension of photo format along flight direction (m)
Application
Computes the spacing between successive exposure stations for a given overlap and ground coverage.
Example
H = 3000 m, p_avg = 92 mm, Δp = 4 mm → Δh = (3000 × 4)/(92 + 4) = 12000/96 = 125 m
Formula
Δh = (H × Δp) / (p_avg + Δp)
Variables
Δh = elevation difference between two points (m); H = flying height above datum (m); Δp = parallax difference between the two points (mm on photo); p_avg = average parallax of the base (mm on photo)
Application
Converts measured x-parallax difference to elevation difference in the stereo model.
Example
B/H = 0.60 → VE ≈ 1.5/0.60 = 2.5× (terrain appears 2.5 times steeper than actual)
Formula
Vertical Exaggeration (VE) ≈ 1.5 / (B/H)
Variables
VE = vertical exaggeration factor (dimensionless); B/H = base-height ratio
Application
Estimates how much steeper terrain appears in the stereo model compared to reality. Lower B/H → higher VE.
Exam Tips
- When given overlap %, immediately compute (1 − overlap) to find the fraction of the ground strip that equals the air-base.
- For parallax problems, identify whether you are given photo parallax (mm) or ground parallax (m) — scale conversions are often required.
- Remember: B/H ≈ 0.6 for standard 60% overlap — use this as a quick check for your computed answers.
- Vertical exaggeration questions may ask for its effect on photointerpretation — VE > 1 means slopes look steeper, aiding recognition of subtle terrain features.
- Board problems often integrate B/H with relief displacement — review Chapter 2 parallax formulas in tandem.
Key Points
- Two overlapping photographs taken from different camera positions provide two slightly different perspectives of the same ground area — the parallax difference between conjugate points encodes elevation.
- Standard forward overlap is approximately 60% (minimum 55%, recommended 60–65%) to guarantee complete stereoscopic coverage between successive exposures along a flight strip.
- Sidelap between adjacent flight strips is typically 30% to prevent gaps in coverage.
- The stereo model is reconstructed by restoring the relative and absolute orientations of the stereopair, allowing three-dimensional measurement of any visible point.
- Vertical exaggeration in a stereo model is controlled by the base-height ratio B/H — the larger the ratio, the greater the apparent vertical relief.
- Parallax is defined as the apparent displacement of a point when seen from the two camera positions; height differences produce x-parallax differences (Δp) that can be measured and converted to elevation differences.
- A pocket stereoscope (lens stereoscope) or mirror stereoscope is used for manual stereo viewing; digital workstations perform stereo matching automatically.
- Stereo compilation is the traditional method for producing topographic maps at NAMRIA — operators extract contours and planimetric features from the stereo model.
Definitions
Term
Parallax
Definition
The apparent shift in position of an object point when viewed from two different camera stations. X-parallax (along flight direction) is used for height measurement; y-parallax (across flight) indicates unresolved model errors or misorientation.
Importance
Foundation of all height determination from stereopairs — directly tested in board exams.
Term
Air-base (B)
Definition
The distance between two successive camera exposure stations along the flight line, measured in object space (metres). It is the baseline of the stereo triangle.
Importance
Used to compute B/H ratio and to determine the air-base from overlap specifications — common board computation.
Term
Base-height ratio (B/H)
Definition
The ratio of the air-base to the flying height above terrain. Controls height accuracy and vertical exaggeration of the stereo model.
Importance
A fundamental design parameter in flight planning — frequently appears in board exam problems.
Term
Forward overlap
Definition
The percentage of a photograph's ground coverage that is also covered by the next photograph in the same flight strip. Standard = 60%.
Importance
Determines the air-base and hence the B/H ratio; also ensures complete stereo coverage.
Term
Sidelap
Definition
The percentage of ground coverage shared between adjacent parallel flight strips. Standard = 30%.
Importance
Prevents gaps in stereoscopic coverage between strips; also affects the number of flight lines required.
Term
Stereo compilation
Definition
The manual or automated process of measuring planimetric and elevation data from a reoriented stereo model to produce topographic maps and feature databases.
Importance
Traditional and still-used workflow at NAMRIA for Philippine topographic map production.
Section Title
Stereoscopy and the Stereo Model
Common Mistakes
- Confusing air-base with photo base — air-base is in ground units (metres); photo base is the image distance between principal points on the photograph (mm).
- Using 60% overlap directly as the air-base fraction — the air-base is the NON-overlapping portion, i.e., (1 − 0.60) = 40% of the ground strip width.
- Ignoring y-parallax — y-parallax does NOT give height; it signals model errors or misorientation.
- Assuming a larger B/H always gives better results — very large B/H increases relief displacement and can cause matching difficulties and occlusions.
- Mixing up flying height above terrain with flying height above sea level — always use height above the terrain (or datum) for B/H and parallax calculations.
Formulas
Example
L = 20 m, A₁ = 15 m², Aₘ = 18 m², A₂ = 21 m² → V = (20/6)(15 + 72 + 21) = (20/6)(108) = 360 m³
Formula
Volume by prismoidal formula: V = (L/6)(A₁ + 4Aₘ + A₂)
Variables
V = volume (m³); L = distance between end cross-sections (m); A₁, A₂ = end cross-section areas (m²); Aₘ = middle cross-section area (m²)
Application
Earthwork volume computation from DEM-derived cross-sections — standard in Philippine road and dam projects.
Example
Standard guidance: CI ≈ H/1000 for accurate photogrammetric compilation (e.g., H = 1500 m → CI ≈ 1.5 m, use 2 m CI)
Formula
Contour interval (CI) ≈ H / (5000 × tan θ_min)
Variables
H = flying height (m); θ_min = minimum detectable slope angle; 5000 = empirical factor
Application
Rule of thumb relating flying height to achievable contour interval in photogrammetric mapping.
Example
Δh = 5 m over Δd = 25 m → slope = (5/25) × 100 = 20%
Formula
Slope (%) = (Δh / Δd) × 100
Variables
Δh = elevation difference between two DEM cells (m); Δd = horizontal distance between cell centres (m)
Application
DEM-derived slope computation for terrain analysis, road alignment, and hazard assessment.
Exam Tips
- Memorise the hierarchy: DSM (all surfaces) → DTM/DEM (bare earth only) — the key distinction tested repeatedly.
- Know the three DEM acquisition methods (photogrammetry, LiDAR, InSAR) and their relative accuracy and coverage characteristics.
- For volume problems from DEMs, practise the prismoidal formula and the average-end-area method — both appear in board exams.
- Remember Phil-LiDAR as the Philippine government's DEM production programme — useful for context-based questions.
- DEM vertical accuracy is expressed as RMSE — for topographic mapping, NAMRIA requires conformance to ASPRS standards which specify accuracy by map scale and class.
Key Points
- A DEM is a digital representation of the bare-earth (ground) surface, stored as a regular grid of elevation values (raster) or as a Triangulated Irregular Network (TIN).
- DEM specifically refers to bare-earth elevations (vegetation and structures removed); DSM (Digital Surface Model) includes all above-ground objects (trees, buildings); DTM (Digital Terrain Model) is often used synonymously with DEM but may also include breaklines and mass points.
- Three primary acquisition methods: (1) photogrammetric image matching from stereopairs, (2) LiDAR (Light Detection And Ranging) — airborne or terrestrial, (3) radar interferometry (InSAR) such as the SRTM dataset.
- Photogrammetric DEMs are produced by automated image matching (area-based or feature-based) in the stereo model — dense point clouds are generated and interpolated to a regular grid.
- LiDAR DEMs are high-accuracy, high-density point clouds; bare-earth DEMs require filtering to remove non-ground returns (vegetation, buildings).
- The SRTM (Shuttle Radar Topography Mission, 2000) provides a global DSM at 1-arc-second (~30 m) resolution — freely available and widely used in the Philippines for regional studies.
- DEM accuracy is expressed as RMSE of elevation (vertical accuracy) — for large-scale Philippine mapping projects, NAMRIA specifies accuracy requirements aligned with ASPRS accuracy standards.
- Key DEM applications: contour generation, volume/earthwork computation, slope and aspect analysis, watershed delineation and drainage modelling, viewshed analysis, orthorectification, flood modelling.
- Grid spacing (resolution) determines the level of terrain detail — finer grid = more detail but larger file size.
- Interpolation methods for DEM generation from point data include bilinear, kriging, IDW (Inverse Distance Weighting), and natural neighbour.
Definitions
Term
DEM (Digital Elevation Model)
Definition
A raster or TIN representation of the bare-earth (ground) surface elevation, with vegetation and man-made structures removed. The most fundamental product of terrain analysis.
Importance
Central product in photogrammetric workflows — feeds orthorectification, contour generation, volume computation, and flood modelling.
Term
DSM (Digital Surface Model)
Definition
A digital model representing the elevation of all visible surfaces including vegetation canopy, buildings, and other above-ground features — essentially what radar or photogrammetry measures directly.
Importance
Contrasted with DEM in board exams — know which is bare-earth (DEM/DTM) and which includes objects (DSM).
Term
DTM (Digital Terrain Model)
Definition
Often used synonymously with DEM; in strict usage, DTM may include additional topographic information such as breaklines, ridge lines, stream networks, and mass points in addition to the elevation grid.
Importance
The distinction between DEM and DTM appears in board exam questions — DTM is the more information-rich product.
Term
LiDAR (Light Detection And Ranging)
Definition
An active remote sensing technology that measures distances by illuminating targets with laser pulses and measuring the time of flight of reflected pulses. Produces high-density 3D point clouds.
Importance
Now the standard tool for high-accuracy DEMs in the Philippines (e.g., Phil-LiDAR programme for flood mapping).
Term
TIN (Triangulated Irregular Network)
Definition
A vector-based terrain representation formed by connecting irregularly spaced elevation points into a network of non-overlapping triangles. Better than a regular grid at representing breaklines and abrupt terrain changes.
Importance
Alternative DEM structure — important for understanding how terrain is stored and computed in GIS/CAD systems.
Term
InSAR (Interferometric Synthetic Aperture Radar)
Definition
A radar technique that uses phase differences between two SAR images acquired from slightly different positions to compute surface elevation — the basis of the global SRTM DEM.
Importance
One of three standard DEM acquisition methods — know its advantages (large area, all-weather) and limitations (DSM, not DEM; penetration depth).
Term
Phil-LiDAR Programme
Definition
A DOST-funded national initiative that produced high-resolution LiDAR DEMs for major river basins and flood-prone areas across the Philippines, primarily for disaster risk reduction.
Importance
Philippine-specific context for DEM production — may appear as a contextual question in board exams.
Section Title
Digital Elevation Models (DEM)
Common Mistakes
- Using DSM for orthorectification instead of DEM — DSM includes tree and building heights, which causes planimetric errors in urban areas; a bare-earth DEM gives the best ortho accuracy.
- Confusing DEM resolution with DEM accuracy — a fine-resolution DEM is not necessarily accurate; accuracy depends on sensor quality and ground control.
- Forgetting that SRTM is a DSM (not a bare-earth DEM) — it includes vegetation and building heights and is not suitable where bare-earth elevations are required.
- Applying the wrong interpolation method — IDW works well for smoothly varying terrain; kriging is preferred when spatial autocorrelation statistics are needed.
- Confusing vertical datum — Philippine maps use the Mean Lower Low Water (MLLW) tidal datum for heights in coastal areas and a MSL-based orthometric datum inland; always confirm the datum when using DEMs.
Formulas
Example
H = 2000 m, h = 50 m (hill), r = 80 mm → d = 50 × 80 / 2000 = 2.0 mm on photo. At scale 1:10 000, this = 20 m ground error.
Formula
Relief displacement: d = h × r / H
Variables
d = radial displacement on photo (mm); h = object height above terrain (m); r = radial distance of object from principal point on photo (mm); H = flying height above terrain (m)
Application
Quantifies the planimetric error in a raw photo due to terrain relief — the error that orthorectification removes.
Example
For well-flown missions, tilt < 3° → tilt displacement is small but non-negligible for large-scale mapping.
Formula
Tilt displacement: δ = r × t × sin(α) / f
Variables
δ = displacement due to tilt (mm); r = radial distance from isocenter (mm); t = tilt angle (radians); α = direction angle from tilt axis; f = focal length (mm)
Application
Estimates image displacement caused by non-vertical camera attitude — also removed by orthorectification.
Example
f = 0.050 m (50 mm lens), pixel size = 0.000006 m (6 μm), H = 500 m → GSD = 0.000006 × (500/0.050) = 0.06 m = 6 cm
Formula
Orthophoto GSD (m) = pixel size (m) on sensor × (H / f)
Variables
GSD = ground sampling distance (m); H = flying height above terrain (m); f = focal length (m)
Application
Determines the spatial resolution of the orthophoto — the fundamental output quality parameter.
Example
GSD = 0.10 m → M = 0.10/0.0002 = 500 → orthophoto supports up to 1:500 scale maps
Formula
Recommended map scale: 1 : M where M = GSD (m) / 0.0002
Variables
M = map scale denominator; GSD = ground sampling distance in metres; 0.0002 m = 0.2 mm minimum detectable feature on a printed map
Application
Links orthophoto pixel resolution to usable map scale for Philippine cadastral and engineering surveys.
Exam Tips
- Board exam problems on orthophotos often ask: 'Why is an orthophoto preferred over a raw photo for measurement?' — answer: uniform scale (relief and tilt removed using DEM).
- Practise computing relief displacement d = h × r / H — given one of the four variables, solve for the others.
- Know the orthophoto production sequence: raw photo → DEM → orthorectification → colour balance → mosaic → final orthomosaic.
- Remember PRS92 / PPCS-UTM as the reference system for Philippine orthophotos — board exams test awareness of the national geodetic framework.
- For UAV-based projects, SfM software (e.g., Agisoft Metashape, Pix4D) produces both DEM and orthomosaic — understand the workflow conceptually.
- GSD formula (GSD = pixel size × H/f) is frequently tested — practise unit conversions (μm to m, mm to m).
Key Points
- A raw aerial photograph is NOT a map — its scale varies with terrain relief (relief displacement) and camera tilt. Measurements of distances and areas from a raw photo are therefore unreliable.
- An orthophoto is produced by differential rectification: the raw photograph is resampled pixel-by-pixel using a DEM so that every image point is shifted to its correct planimetric position, removing both relief displacement and tilt effects.
- The result is an image at a uniform scale — it can be used as a base map and distances/areas can be measured directly, just like a topographic map.
- Orthorectification is the specific process of differential rectification using a DEM; simple rectification (for flat terrain) removes only tilt.
- An orthomosaic is a seamless mosaic of multiple orthophotos, typically colour-balanced and feathered at seam lines, covering an entire project area.
- Ground Control Points (GCPs) are needed for absolute orientation — they tie the photo coordinate system to a geodetic reference frame (PRS92 / WGS84 in the Philippines).
- Check points (independent from GCPs) are used to validate the accuracy of the orthophoto — their residuals give the RMSE of planimetric accuracy.
- Orthophotos can be produced from any imaging sensor — aerial film, digital frame cameras, UAV cameras, or satellite imagery — as long as a DEM is available.
- In the Philippines, NAMRIA produces the official orthophotomaps; private geodetic firms produce project-specific orthophotos under PD 1529 and related laws.
- For UAV-based mapping (now common in Philippine cadastral and engineering surveys), Structure-from-Motion (SfM) photogrammetry produces both the DEM and orthomosaic simultaneously.
- The minimum recommended GCP configuration is 4 GCPs for a small project block (one at each corner); larger projects require a systematic distribution.
- Pixel size (Ground Sampling Distance, GSD) of the orthophoto determines its usable map scale: map scale denominator ≈ GSD (mm) × 1000 / 0.2 (assuming 0.2 mm map detail threshold).
Definitions
Term
Orthophoto
Definition
A geometrically corrected aerial or satellite photograph in which the effects of camera tilt and terrain relief have been removed by differential rectification using a DEM, resulting in a uniform-scale image that can be used as a planimetric base map.
Importance
The primary deliverable of most modern photogrammetric mapping projects — tested directly in board exams.
Term
Differential rectification (Orthorectification)
Definition
The process of correcting a raw photograph pixel-by-pixel using a DEM, shifting each pixel to its correct ground position by accounting for both tilt and relief displacement simultaneously.
Importance
Distinguishes orthorectification from simple rectification (tilt only) — an important distinction in board exam questions.
Term
Relief displacement
Definition
The radial outward shift of the image of an elevated object from the position it would occupy if the object were at datum elevation, caused by the perspective geometry of aerial photography. It increases with distance from the principal point and with object height.
Importance
The primary reason a raw photo cannot be used as a map — must understand its direction and magnitude.
Term
Ground Control Points (GCPs)
Definition
Identifiable points on the ground whose planimetric coordinates (Northing, Easting in PRS92/PPCS-UTM) and elevation are known from geodetic surveys, used to orient the photogrammetric model to the national reference frame.
Importance
Essential for producing legally admissible maps in the Philippines under PD 1529 — accuracy depends on GCP quality and distribution.
Term
Orthomosaic
Definition
A seamless, radiometrically balanced image produced by mosaicking and blending multiple individual orthophotos into a single continuous image covering the entire project area.
Importance
The final deliverable in most mapping projects — may be tested in terms of production steps or accuracy requirements.
Term
Structure-from-Motion (SfM)
Definition
A photogrammetric technique that simultaneously recovers camera orientations, sparse 3D point clouds, dense DEMs, and orthomosaics from sets of overlapping images with no pre-known camera positions, widely used in UAV surveys.
Importance
Now the dominant method for small-area photogrammetric surveys in the Philippines — understand its workflow and limitations.
Term
Ground Sampling Distance (GSD)
Definition
The ground dimension represented by one pixel in an aerial or satellite image — the fundamental measure of image spatial resolution. Smaller GSD = finer detail.
Importance
Determines achievable map scale and is a key flight planning parameter — common in board exam computations.
Section Title
Orthophoto and Orthomosaic Production
Common Mistakes
- Treating a raw photo as a map — raw photos have scale that varies with relief; only orthophotos have uniform scale suitable for measurement.
- Using a DSM instead of a DEM for orthorectification in urban areas — DSM shifts building tops to correct position but leaves building bases in wrong position, creating the 'keystone effect'.
- Confusing rectification with orthorectification — simple rectification removes only tilt (assumes flat terrain); orthorectification also removes relief displacement using a DEM.
- Computing orthophoto accuracy from GCPs instead of independent check points — GCP residuals are not independent; always use separate check points for accuracy assessment.
- Forgetting that PRS92 (ITRF92 realisation) is the national geodetic reference frame for the Philippines — GCPs must be referenced to PRS92 for NAMRIA-compliant deliverables.
Exam Tips
- Expect at least one question per board exam on PRS92, PPCS/UTM, or the legal framework (RA 8560, PD 1529) — review these briefly but accurately.
- Know that NAMRIA produces the official Philippine topographic maps (1:10 000 to 1:250 000 series) and that these are based on photogrammetric compilation from aerial photographs.
- RA 8560 scope question: photogrammetric surveys ARE within the scope of geodetic engineering practice in the Philippines.
Key Points
- RA 4374 (as amended by RA 8560) governs the practice of Geodetic Engineering in the Philippines — photogrammetric surveys and map production are among the defined scopes of practice.
- PD 1529 (Property Registration Decree) requires that all cadastral surveys and plans submitted for land registration be prepared by licensed Geodetic Engineers and conform to DENR-LMB technical standards.
- CA 141 (Public Land Act) governs the classification and disposition of public lands — requires accurate surveys whose products include photogrammetrically produced plans.
- NAMRIA (National Mapping and Resource Information Authority) is the central mapping agency of the Philippines under DND — responsible for producing official topographic maps, orthophotomaps, and DEMs at national scale.
- PRS92 (Philippine Reference System of 1992) is the national geodetic reference system — based on ITRF92 and compatible with WGS84 to within ±1 m. All official Philippine maps and surveys must be referenced to PRS92.
- PPCS (Philippine Plane Coordinate System) uses the UTM projection in three zones covering the Philippines (Zone 50N, 51N, 52N) with PRS92 as the geodetic datum.
- The Phil-LiDAR project (2014–2017, DOST) produced 1-m resolution LiDAR DEMs for over 25 major river basins — these DEMs are now the standard for flood hazard mapping in the Philippines.
- UAV surveys in the Philippines require CAB (Civil Aeronautics Board) and CAAP (Civil Aviation Authority of the Philippines) clearances for commercial aerial photography.
Definitions
Term
PRS92
Definition
Philippine Reference System of 1992 — the national geodetic reference frame, realised as the ITRF92 ellipsoid (GRS80/WGS84-compatible) with coordinates in geographic (latitude/longitude) or PPCS/UTM (Northing/Easting).
Importance
All official Philippine surveys, maps, and orthophotos must be referenced to PRS92 — tested as a context question in board exams.
Term
PPCS/UTM
Definition
Philippine Plane Coordinate System using the Universal Transverse Mercator projection on the PRS92 datum. The Philippines falls in UTM Zones 50N, 51N, and 52N, with the Easting origin at 500 000 m E for each zone's central meridian.
Importance
The standard projected coordinate system for Philippine engineering surveys and photogrammetric products — coordinates appear in all project deliverables.
Term
NAMRIA
Definition
National Mapping and Resource Information Authority — the official national mapping agency responsible for topographic mapping, geodetic control, hydrographic surveying, and the national DEM of the Philippines.
Importance
The primary government agency for official photogrammetric map production — referenced in board exam legal/institutional questions.
Section Title
Philippine Legal and Institutional Context
Common Mistakes
- Assuming WGS84 = PRS92 — they are close (within ~1 m) but not identical; official Philippine deliverables must explicitly reference PRS92.
- Submitting photogrammetric plans signed by non-licensed personnel — RA 8560 requires a licensed Geodetic Engineer to sign all survey plans.
- Confusing NAMRIA's role (national base mapping) with DENR-LMB's role (cadastral survey administration and land registration support).
Connections
- Stereoscopy links directly to Chapter 2 (Parallax and Relief Displacement) — the parallax difference formula (Δh = H·Δp/p_avg) is applied within the stereo model to compute terrain elevations that populate the DEM.
- DEM production connects to flight planning (Chapter 1) — the required DEM accuracy and contour interval determine the flying height, photo scale, and overlap specifications for the aerial survey.
- Orthophoto production depends on both the stereo model (for the DEM) and the camera model (interior orientation, lens distortion) — all camera calibration parameters from Chapter 1 feed into the orthorectification process.
- GCPs and absolute orientation (Chapter 3) are prerequisite to orthophoto production — without geodetic control in PRS92/PPCS-UTM, the orthophoto has no defined planimetric position.
- Volume computation from DEMs (engineering surveys) connects to Surveying and Levelling courses — the prismoidal and average-end-area formulas are used identically whether cross-sections come from field levelling or DEM extraction.
- Legal context: PD 1529 and RA 8560 frame the professional responsibility of the geodetic engineer in producing orthophotos and DEMs for land registration and cadastral purposes — connecting this chapter to Professional Practice and Ethics.
- Remote sensing (satellite imagery orthorectification) is a direct extension — the same differential rectification principles apply to SPOT, Landsat, and WorldView imagery, connecting Photogrammetry to Remote Sensing subjects.
- GIS applications: DEMs and orthophotos are the primary inputs to GIS-based hazard mapping (flood, landslide) — connecting Photogrammetry to Geographic Information Systems and Cartography.
Exam Strategy
For the PRC Geodetic Engineer board examination in Photogrammetry and Cartography, approach this chapter with a three-tier strategy. TIER 1 — Computation problems (highest weight): Master the four key formulas: (1) B/H = air-base/H, (2) air-base = (1 − overlap) × ground strip width, (3) relief displacement d = h·r/H, and (4) GSD = pixel size × H/f. Practice solving for any variable given the others. Always check units (convert mm to m) and verify your B/H ≈ 0.6 for standard 60% overlap as a sanity check. TIER 2 — Conceptual distinctions (medium weight): Memorise the exact differences between DEM vs DSM vs DTM, orthophoto vs raw photo vs rectified photo, and orthorectification vs simple rectification. These appear as MCQ identification questions. TIER 3 — Philippine context (lower weight but frequent): Know PRS92/PPCS-UTM, NAMRIA, Phil-LiDAR, RA 8560, and PD 1529 at a recognition level — enough to identify the correct legal/institutional answer in MCQs. Time management tip: computation questions (Tier 1) require the most time — allocate 3–4 minutes each. Conceptual MCQs (Tier 2 and 3) should take 1–2 minutes. If a computation seems overly complex, check whether a simpler formula directly applies before expanding — board problems are typically solvable in 3–5 steps.
Quick Review Questions
Successive aerial exposures are 800 m apart and the flying height is 2000 m. What is the base-height ratio?
B/H = air-base ÷ flying height = 800 ÷ 2000 = 0.40. Note this is lower than the standard 0.60 for 60% overlap, indicating either greater overlap (>60%) or a less favourable height geometry. For 60% overlap, B/H ≈ 0.60.
For a photo at scale 1:8 000 with format 230 mm and 65% forward overlap, compute the air-base.
Ground dimension of photo strip: 0.230 m × 8 000 = 1 840 m. Non-overlapping fraction: 1 − 0.65 = 0.35. Air-base = 0.35 × 1 840 = 644 m.
State the key difference between a DEM and a DSM.
This distinction is critical: SRTM is a DSM; LiDAR-derived bare-earth products after ground filtering are DEMs. For orthorectification of urban areas, a DEM is preferred because a DSM causes 'leaning building' artefacts.
Why cannot a raw aerial photograph be used as a planimetric base map?
Only after differential rectification using a DEM (producing an orthophoto) is the scale uniform and measurement possible. This is the fundamental justification for orthophoto production.
An aerial camera has a focal length of 152 mm and a pixel size of 9 μm. At a flying height of 1 500 m above terrain, compute the GSD.
GSD = pixel size × (H/f) = 9×10⁻⁶ m × (1500/0.152) = 9×10⁻⁶ × 9868 = 0.0888 m. This GSD supports maps up to approximately 1:444 scale (= 0.0888/0.0002).
A building 40 m tall is imaged at a radial distance of 60 mm from the principal point on a photo taken at H = 1 200 m. What is the relief displacement of the building top?
d = h × r / H = 40 × 60 / 1200 = 2 400/1 200 = 2.0 mm. This is the distance by which the building top is displaced radially outward from where it would appear at ground level. Orthorectification removes this displacement.
List three data sources for producing a DEM.
Each method has trade-offs: photogrammetry requires overlap and texture; LiDAR is high accuracy but expensive; InSAR covers large areas rapidly but produces DSM and has lower accuracy in forested areas. All three are standard exam topics.
What reference system and projection are used for Philippine orthophoto deliverables?
PRS92 is the mandatory national geodetic reference frame under NAMRIA. It is compatible with WGS84/ITRF92. All coordinates in official Philippine maps and orthophotos are expressed in PRS92/PPCS-UTM.
What is the effect of increasing the base-height ratio (B/H) on the stereo model?
B/H is a design trade-off: too small → poor height geometry; too large → excessive relief displacement and occlusions. The standard B/H ≈ 0.6 for 60% overlap represents the optimal balance for most topographic mapping.
Define orthorectification and distinguish it from simple rectification.
The critical distinction: simple rectification uses a planar transformation (no DEM needed); orthorectification requires a DEM and applies a different correction to each pixel based on local terrain elevation. Board exams frequently test this distinction.
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