GELE Photogrammetry & Cartography — Stereoscopy, DEM and OrthophotoExam Answer Templates

Marks

1

Topic

Stereoscopy

Difficulty

easy

Template Id

T1

Examiner Tip

One precise sentence is enough for 1 mark. The key trigger words are 'overlapping,' 'two positions,' and '3-D model' or 'parallax.' Include at least two of these.

Model Answer

Stereoscopy is the technique of viewing two overlapping photographs of the same ground area from slightly different camera positions to create a three-dimensional (3-D) perception of the terrain, enabling the measurement of object heights through parallax.

Question Type

very_short_answer

Answer Structure

• One sentence: Name the technique AND state its purpose (3-D perception/height measurement) [1 mark]

Scoring Breakdown

Common Mark Deductions

• Writing only 'viewing two photos in 3-D' without mentioning the measurement or parallax aspect
• Confusing stereoscopy with photogrammetry in general
• Omitting the idea of two different viewpoints

Key Phrases To Include

• overlapping photographs
• three-dimensional model
• different camera positions
• parallax
• height measurement

Marks

1

Topic

Base-Height Ratio

Difficulty

easy

Template Id

T2

Examiner Tip

For 1-mark definition questions, always include both the definition AND the practical significance in one sentence to guarantee the mark.

Model Answer

The base-height ratio (B/H) is the ratio of the air-base (distance between successive exposure stations) to the flying height above datum. It controls the geometric strength of height determination: a larger B/H gives stronger (more accurate) vertical measurements.

Question Type

very_short_answer

Answer Structure

• Define B/H (ratio of air-base to flying height) AND state its significance (height measurement strength) [1 mark]

Scoring Breakdown

Common Mark Deductions

• Defining B/H but not explaining its significance
• Reversing the ratio (H/B instead of B/H)
• Stating flying height above sea level instead of above datum or ground

Key Phrases To Include

• air-base
• flying height
• geometric strength
• height determination
• vertical exaggeration

Marks

1

Topic

Orthophoto

Difficulty

easy

Template Id

T3

Examiner Tip

The word 'uniform scale' or 'constant scale' is the strongest keyword. Examiners award the mark when they see that the student understands the orthophoto behaves like a map.

Model Answer

An orthophoto is an aerial photograph that has been geometrically corrected (orthorectified) for relief displacement and camera tilt using a Digital Elevation Model (DEM), resulting in a uniform scale image that can be measured like a map.

Question Type

very_short_answer

Answer Structure

• State what it is (corrected/orthorectified photo) AND what was corrected (relief displacement, tilt) AND the result (uniform scale) [1 mark]

Scoring Breakdown

Common Mark Deductions

• Describing it only as a 'corrected photo' without specifying what distortions are removed
• Not mentioning uniform scale or the role of the DEM
• Confusing orthophoto with a raw aerial photo

Key Phrases To Include

• orthorectified
• relief displacement
• uniform scale
• DEM
• geometrically corrected
• measurable like a map

Marks

2

Topic

Digital Elevation Model

Difficulty

easy

Template Id

T4

Examiner Tip

Use a parallel structure: 'DEM = ... ; DSM = ...' Examiners mark one mark per correct model definition. The summary 'key difference' line is not required but shows mastery.

Model Answer

A Digital Elevation Model (DEM), also called a Digital Terrain Model (DTM), represents the bare-earth surface — it includes only the ground elevations, with vegetation, buildings, and other above-ground features removed. A Digital Surface Model (DSM) represents the top surface of all features in the landscape, including buildings, trees, power lines, and other objects above the ground, in addition to the terrain. Key difference: DEM/DTM = bare earth; DSM = earth plus all above-ground objects.

Question Type

short_answer

Answer Structure

• Line 1–2: Define DEM/DTM — bare-earth elevation, objects removed [1 mark]
• Line 3–4: Define DSM — includes above-ground features (buildings, trees) [1 mark]

Scoring Breakdown

Common Mark Deductions

• Defining only one model and not the other
• Saying DEM includes buildings — it does NOT
• Using 'surface' and 'elevation' interchangeably without distinguishing what each model represents
• Omitting the key difference statement

Key Phrases To Include

• bare-earth
• bare-ground
• terrain
• above-ground features
• buildings
• vegetation
• Digital Terrain Model (DTM)

Marks

2

Topic

Orthophoto

Difficulty

medium

Template Id

T5

Examiner Tip

When a question mentions 'cadastral mapping,' connect your answer to RA 8560 or PD 1529 for bonus context. Examiners appreciate domain-specific framing.

Model Answer

1. Uniform, measurable scale: An orthophoto has a constant planimetric scale throughout because relief displacement and tilt have been removed using a DEM, allowing direct measurement of distances and areas — a critical requirement under PD 1529 (Property Registration Decree) for cadastral maps. 2. Geometric accuracy: Boundaries, lot corners, and features are in their correct planimetric positions, making the orthophoto suitable as a base map for cadastral surveys without introducing errors due to terrain relief.

Question Type

short_answer

Answer Structure

• Advantage 1: Uniform/constant scale for measurement (with reason: relief displacement removed) [1 mark]
• Advantage 2: Geometric accuracy / correct planimetric positions of features [1 mark]

Scoring Breakdown

Common Mark Deductions

• Listing the same advantage twice in different words
• Saying 'better quality image' without explaining why — image clarity is not the primary advantage
• Not connecting the answer to the photogrammetric correction process

Key Phrases To Include

• uniform scale
• relief displacement removed
• planimetric position
• measurable
• geometric accuracy
• base map

Marks

2

Topic

Base-Height Ratio

Difficulty

easy

Template Id

T6

Examiner Tip

In numerical problems, the examiner follows a mark scheme: formula = 1 mark, correct answer = 1 mark. Writing the formula even if you make an arithmetic error saves at least 1 mark.

Model Answer

Given: Air-base, B = 900 m Flying height, H = 1,500 m Formula: B/H = Air-base / Flying height Solution: B/H = 900 / 1,500 B/H = 0.60 Answer: The base-height ratio is 0.60 (dimensionless). This value (~0.6) is typical for 60% forward overlap and indicates good geometric strength for height determination.

Question Type

numerical

Answer Structure

• Write the Given data (B and H with units) [½ mark implied]
• Write the Formula: B/H = B/H [½ mark implied]
• Correct substitution and computation [1 mark]
• Correct final answer with interpretation [1 mark]

Scoring Breakdown

Common Mark Deductions

• Inverting the ratio (computing H/B = 1.667 instead of B/H = 0.60)
• Including units for B/H — it is dimensionless
• Skipping the formula and going directly to the answer
• Using inconsistent units (mixing metres and kilometres)

Key Phrases To Include

• air-base
• flying height
• B/H = 0.60
• dimensionless
• geometric strength

Marks

2

Topic

Stereoscopy / Air-base

Difficulty

medium

Template Id

T7

Examiner Tip

The most common error in this type of problem is using 60% instead of 40%. Remember: overlap means the photos SHARE that portion, so the aircraft advances only the NON-overlapping (40%) portion per exposure.

Model Answer

Given: Forward overlap = 60% Photo format = 230 mm = 0.230 m Photo scale denominator = 10,000 Step 1 — Ground dimension covered by one photo (in the flight direction): Ground side = format × scale denominator Ground side = 0.230 m × 10,000 = 2,300 m Step 2 — Air-base (distance between successive exposure stations): With 60% overlap, the non-overlapping portion = 100% − 60% = 40% Air-base = 40% × ground side Air-base = 0.40 × 2,300 m Air-base = 920 m Answer: The air-base is 920 m.

Question Type

numerical

Answer Structure

• Step 1: Convert photo format to ground distance using scale [1 mark]
• Step 2: Apply overlap percentage to find air-base [1 mark]

Scoring Breakdown

Common Mark Deductions

• Using 60% instead of 40% (the non-overlapping fraction) to compute the air-base
• Not converting 230 mm to 0.230 m before multiplying by scale denominator
• Forgetting to multiply format by scale denominator to get ground dimension

Key Phrases To Include

• ground side
• scale denominator
• non-overlapping portion
• 40%
• air-base = 920 m

Marks

3

Topic

Orthophoto / Relief Displacement

Difficulty

medium

Template Id

T8

Examiner Tip

A 3-mark question expects three distinct, developed points. Do not give three one-word answers — each point must include the mechanism (WHY it happens). Examiners look for 'relief displacement,' 'tilt,' and 'central perspective' as signal words.

Model Answer

A raw aerial photograph cannot be used directly as a map for the following reasons: 1. Variable scale due to relief displacement: The scale of a raw photograph varies across the image because elevated terrain features are displaced radially outward from the photo nadir. Objects on high ground appear farther from their true planimetric position than objects at lower elevations. This makes direct distance or area measurement inaccurate. 2. Scale variation due to camera tilt: Even a small tilt of the aerial camera from the vertical changes the scale across the photo, making one side appear at a different scale than the other. This introduces positional errors in feature locations. 3. Central perspective geometry: A raw photo is a central (perspective) projection, not an orthogonal projection. It records radial distances from the principal point, not true horizontal distances. Only an orthophoto — produced by removing relief and tilt distortions using a DEM — has a uniform, orthogonal scale suitable for cadastral or boundary surveys under PD 1529.

Question Type

short_answer

Answer Structure

• Point 1: Variable scale due to relief displacement — explain the mechanism [1 mark]
• Point 2: Scale variation due to tilt — briefly explain [1 mark]
• Point 3: Central perspective geometry vs orthogonal projection — link to need for orthophoto/DEM [1 mark]

Scoring Breakdown

Common Mark Deductions

• Saying only 'the photo is distorted' without specifying what causes the distortion
• Not mentioning relief displacement by name
• Failing to connect the solution (orthophoto/DEM) to the problem
• Writing a very short answer with only one reason for a 3-mark question

Key Phrases To Include

• relief displacement
• variable scale
• camera tilt
• central perspective projection
• orthogonal projection
• orthorectification
• DEM
• uniform scale

Marks

3

Topic

Digital Elevation Model

Difficulty

medium

Template Id

T9

Examiner Tip

For 'list and describe' questions, each item earns 1 mark for the correct name PLUS a correct brief description. The name alone is usually insufficient. Write at least one sentence of explanation per method.

Model Answer

Three methods of producing a DEM: 1. Photogrammetric image matching: Overlapping stereo aerial or satellite images are processed using automated image-matching algorithms (dense matching) to extract the 3-D coordinates of terrain points. The matched points are interpolated into a regular grid DEM. 2. LiDAR (Light Detection and Ranging): An airborne or terrestrial laser scanner emits laser pulses toward the ground and measures the time-of-flight of the return signal. The x, y, z coordinates of millions of ground points (point cloud) are computed and filtered to produce a bare-earth DEM. 3. Radar interferometry (InSAR — Interferometric Synthetic Aperture Radar): Two SAR (Synthetic Aperture Radar) images acquired from slightly different orbital positions are processed interferometrically to extract phase differences proportional to terrain elevation, producing a DEM from satellite radar data (e.g., SRTM, TanDEM-X).

Question Type

short_answer

Answer Structure

• Method 1: Photogrammetric image matching — name it and describe the principle [1 mark]
• Method 2: LiDAR — name it and describe the principle [1 mark]
• Method 3: InSAR/Radar — name it and describe the principle [1 mark]

Scoring Breakdown

Common Mark Deductions

• Listing 'GPS' as a DEM production method without specifying that GPS is used for ground control, not direct DEM generation
• Naming the method but not describing how it works — examiners require the mechanism
• Listing photogrammetry twice (e.g., satellite and aerial) as two separate methods without a third distinct method

Key Phrases To Include

• stereo image matching
• LiDAR
• laser pulses
• point cloud
• InSAR
• Interferometric SAR
• phase difference
• SRTM

Marks

3

Topic

Base-Height Ratio / Stereoscopy

Difficulty

hard

Template Id

T10

Examiner Tip

Part (b) is worth 2 marks — one for the correct action and one for the explanation. Always answer BOTH the 'what' and the 'why' for multi-mark sub-questions.

Model Answer

Given: Flying height, H = 3,000 m Air-base, B = 1,200 m (a) Base-height ratio: B/H = B / H = 1,200 / 3,000 = 0.40 The base-height ratio is 0.40. (b) To increase B/H, the flight plan can be adjusted by reducing the forward overlap (e.g., from 60% to 50% or lower). Explanation: The air-base B equals the non-overlapping fraction of the ground coverage per photo. Reducing the forward overlap increases the non-overlapping portion, which increases the air-base B for the same flying height H. A larger B/H provides stronger (more precise) height determination in the stereo model. However, the trade-off is that reducing overlap may approach the minimum acceptable stereo overlap, and terrain occlusions (dead ground) may appear, especially in the mountainous Cordillera terrain. Standard practice requires at least 55–60% forward overlap for reliable stereoscopy.

Question Type

numerical

Answer Structure

• Part (a): Formula + correct substitution + answer B/H = 0.40 [1 mark]
• Part (b): Correct adjustment stated (reduce forward overlap / increase air-base) [1 mark]
• Part (b): Correct explanation of the mechanism and the trade-off [1 mark]

Scoring Breakdown

Common Mark Deductions

• Computing B/H = 3,000/1,200 = 2.5 (inverted ratio)
• Suggesting to reduce flying height to increase B/H without recognizing that this changes ground coverage, not just the ratio
• Not explaining the mechanism of how reducing overlap increases the air-base

Key Phrases To Include

• B/H = 0.40
• reduce forward overlap
• increase air-base
• non-overlapping fraction
• stronger height determination
• 55-60% minimum overlap

Marks

3

Topic

Orthophoto / Orthorectification

Difficulty

medium

Template Id

T11

Examiner Tip

The word 'DEM' must appear in your answer for this question — it is non-negotiable. Examiners mark it as a required term. Also, always name both distortions: relief displacement AND tilt.

Model Answer

Orthorectification is the process of geometrically correcting a raw aerial photograph to remove distortions caused by (1) terrain relief and (2) camera tilt, producing an orthophoto with uniform, map-consistent planimetric scale. Process: 1. DEM input: A Digital Elevation Model (DEM) of the survey area is used to model the actual terrain surface. For each pixel in the raw photo, the DEM provides the ground elevation at that location. 2. Pixel reprojection: Using the known camera parameters (interior orientation — focal length, principal point, lens distortion) and exterior orientation (position and attitude of the camera at exposure), each raw image pixel is mathematically reprojected to its correct orthogonal ground position, accounting for the elevation given by the DEM. 3. Resampling: The pixel grey values (or RGB values) are resampled (using bilinear or cubic interpolation) to fill the corrected output grid, removing the positional error caused by relief displacement and tilt. Distortions corrected: - Relief displacement: radial outward shift of elevated objects from their true planimetric position - Tilt distortion: scale variation across the photo due to non-vertical camera orientation Result: An orthophoto with uniform scale that can be used as a planimetric map base — suitable for cadastral surveys, land-use mapping, and orthomosaic production.

Question Type

short_answer

Answer Structure

• Step 1: Role of DEM — provides ground elevation for each pixel [1 mark]
• Step 2: Reprojection using interior and exterior orientation [1 mark]
• Step 3: Type of distortions corrected (relief displacement + tilt) [1 mark]

Scoring Breakdown

Common Mark Deductions

• Describing orthorectification as simply 'correcting the photo' without specifying what is corrected or the role of the DEM
• Mentioning only relief displacement and omitting tilt
• Confusing orthorectification with radiometric correction or image enhancement

Key Phrases To Include

• DEM
• relief displacement
• tilt distortion
• reprojection
• resampling
• interior orientation
• exterior orientation
• uniform scale
• planimetric position

Marks

5

Topic

Air-base / Strip Spacing / Base-Height Ratio

Difficulty

hard

Template Id

T12

Examiner Tip

For 5-mark numericals, organize your solution clearly into labeled steps. Examiners follow your work line by line. Even if one step is wrong, subsequent correctly-applied steps can still earn marks (follow-through marking). Never skip the 'Given' section.

Model Answer

Given: Format size = 230 mm × 230 mm Photo scale denominator = 8,000 (scale = 1:8,000) Forward overlap = 65% Sidelap = 30% Focal length, f = 152 mm Step 1 — Ground dimension covered by one photo (both directions): Ground side = format × scale denominator Ground side = 0.230 m × 8,000 = 1,840 m (Same in both x and y for a square format) (a) Air-base (along-track spacing between exposures): Non-overlapping fraction (forward) = 1 − 0.65 = 0.35 Air-base, B = 0.35 × 1,840 m Air-base, B = 644 m (b) Strip spacing / distance between flight lines (across-track): Non-overlapping fraction (side) = 1 − 0.30 = 0.70 Strip spacing = 0.70 × 1,840 m Strip spacing = 1,288 m (c) Flying height H from photo scale: Scale = f / H → H = f / Scale H = 0.152 m / (1/8,000) H = 0.152 m × 8,000 H = 1,216 m Base-height ratio: B/H = 644 / 1,216 B/H = 0.529 ≈ 0.53 Summary of Answers: (a) Air-base = 644 m (b) Strip spacing = 1,288 m (c) B/H = 0.53 (dimensionless)

Question Type

numerical

Answer Structure

• Step 1: Ground dimension = format × scale denominator = 1,840 m [1 mark]
• Part (a): Air-base = (1 − 0.65) × 1,840 = 644 m [1 mark]
• Part (b): Strip spacing = (1 − 0.30) × 1,840 = 1,288 m [1 mark]
• Part (c): Flying height H = f × scale denominator = 1,216 m [1 mark]
• Part (c): B/H = 644/1,216 = 0.53 [1 mark]

Scoring Breakdown

Common Mark Deductions

• Using 65% (instead of 35%) for air-base and 30% (instead of 70%) for strip spacing
• Not converting 230 mm to 0.230 m (leading to an answer in mm × 8,000 = 1,840,000 mm instead of 1,840 m)
• Computing scale as H/f instead of f/H, getting the wrong flying height
• Forgetting to compute B/H after finding H (partial completion of part c)

Key Phrases To Include

• ground side = format × scale denominator
• non-overlapping fraction
• air-base = 644 m
• strip spacing = 1,288 m
• H = f × scale denominator
• B/H = 0.53

Marks

5

Topic

Digital Elevation Model — Applications

Difficulty

hard

Template Id

T13

Examiner Tip

For 5-mark essay questions, think '1 mark per developed point.' Reference Philippine agencies (NAMRIA, PHIVOLCS, DOST) and laws (PD 1529, RA 10121, CA 141) to show applied knowledge — examiners award bonus credit for domain-specific context even when not explicitly required.

Model Answer

A Digital Elevation Model (DEM) is a digital representation of terrain elevation stored as a regular grid or Triangulated Irregular Network (TIN). In Philippine geodetic engineering practice, DEMs are fundamental to the following applications: 1. Orthophoto Production and Topographic Mapping (1 mark): A DEM is the essential input for orthorectification — the process of removing relief displacement and tilt from raw aerial or satellite images to produce orthophotos. These orthophotos serve as the planimetric base for topographic maps produced by agencies such as NAMRIA (National Mapping and Resource Information Authority), which is mandated to produce official topographic maps of the Philippines under EO 29. 2. Contour Generation and Terrain Analysis (1 mark): DEMs are used to automatically generate contour lines at specified intervals (e.g., 20 m contours for 1:50,000 NAMRIA toposheets). They also support slope, aspect, hillshade, and terrain profile computations needed in engineering feasibility studies, road design, and dam site investigations. 3. Hydrological Modeling and Flood Mapping (1 mark): DEMs drive watershed delineation, stream network extraction, and flow accumulation analyses. In a typhoon-prone country like the Philippines, DEM-based flood inundation modeling is used by PHIVOLCS, DOST-PAGASA, and LGUs for disaster risk reduction and management (DRRM) planning — a direct application of RA 10121 (DRRM Act of 2010). 4. Volume and Earthwork Computation (1 mark): In infrastructure projects (highways, dams, reclamation), DEMs from pre- and post-construction surveys are differenced (cut-and-fill analysis) to compute earthwork volumes, which is essential for quantity estimation under government construction projects. 5. Cadastral and Land Administration Support (1 mark): Under PD 1529 (Property Registration Decree) and CA 141 (Public Land Act), accurate terrain data is needed for lot subdivision, boundary monument placement, and the adjudication of public lands in mountainous areas. DEMs support the correction of cadastral boundaries distorted by relief in remote sensing imagery. Conclusion: The DEM is the foundational dataset for modern geodetic engineering workflows in the Philippines, linking photogrammetric data acquisition to map production, infrastructure engineering, disaster management, and land administration.

Question Type

long_answer

Answer Structure

• Introduction: Define DEM clearly [½ mark]
• Application 1: Orthophoto / topographic mapping with NAMRIA reference [1 mark]
• Application 2: Contour generation / terrain analysis [1 mark]
• Application 3: Hydrological modeling / flood mapping with RA 10121 or DOST reference [1 mark]
• Application 4: Volume / earthwork computation [1 mark]
• Application 5 or Conclusion: Cadastral / land admin or synthesis [½ mark]

Scoring Breakdown

Common Mark Deductions

• Listing applications without describing how the DEM is used in each — 'DEM is used in surveying' earns zero marks
• Covering fewer than three applications for a 5-mark question
• Writing a very general answer without any Philippine-specific agencies, laws, or context
• Not defining DEM at the start of the answer

Key Phrases To Include

• orthorectification
• NAMRIA
• contour generation
• watershed delineation
• flood inundation modeling
• earthwork volume
• PD 1529
• RA 10121
• TIN
• terrain analysis

Marks

5

Topic

Stereoscopy / Base-Height Ratio / Air-base

Difficulty

hard

Template Id

T14

Examiner Tip

This is a conceptual 5-mark question. Examiners reward structured, connected answers. Use numbered sections or clear paragraphs — do not write a continuous paragraph. Show the formula B = (1 − p) × L even if not explicitly asked, as it demonstrates deep understanding.

Model Answer

In aerial photogrammetry, forward overlap, air-base, and base-height ratio are interrelated flight parameters that collectively govern the quality and geometric strength of the stereoscopic model. 1. Forward Overlap (p%) — Definition and Role (1 mark): Forward overlap is the percentage of the ground area common to two successive photographs along a flight strip. Standard practice requires approximately 60% forward overlap to ensure every ground point is covered by at least two photos, forming a stereo pair. A minimum of ~55% is required even in flat terrain; 80% overlap is used for dense image matching in UAV photogrammetry. 2. Air-base (B) — Relationship to Overlap (1 mark): The air-base is the horizontal distance between two successive exposure stations. It is directly related to forward overlap: B = (1 − p/100) × L where L is the ground dimension of the photo in the flight direction and p is the overlap percentage. A smaller overlap → larger air-base → the two photos look at the terrain from more widely separated positions → stronger parallax differences for the same relief → better vertical discrimination. 3. Base-Height Ratio (B/H) — Geometric Strength (1 mark): The base-height ratio B/H = air-base / flying height controls the sensitivity of height measurement. A larger B/H means: - Greater parallax differences between stereo pair images for the same height difference - More precise height determination - Greater vertical exaggeration in the stereo model (terrain relief appears amplified) Typically, B/H ≈ 0.6 for 60% overlap with standard formats, which is a good balance between coverage efficiency and height accuracy. 4. Effect on Stereoscopic Model Quality (1 mark): - Too small B/H (high overlap, small air-base): Height measurement is imprecise — small parallax differences are difficult to measure accurately. The model appears 'flat.' - Optimal B/H (~0.6–0.8): Good balance of coverage and height precision. - Too large B/H (low overlap, large air-base): Strong height determination but risk of 'dead ground' (terrain hidden from one camera position) especially in rugged Philippine topography (e.g., Cordillera, Mt. Apo). 5. Practical Implication — Summary (1 mark): The flight planner balances overlap (coverage safety), air-base (geometric strength), and B/H (height quality). In practice, NAMRIA and private geodetic firms in the Philippines specify 60% ± 5% forward overlap as the standard, yielding B/H ≈ 0.55–0.65 — sufficient for DEM generation at scales up to 1:5,000.

Question Type

long_answer

Answer Structure

• Section 1: Define forward overlap and its standard value [1 mark]
• Section 2: Define air-base and express its relationship to overlap using formula [1 mark]
• Section 3: Define B/H ratio and explain its role in height measurement strength [1 mark]
• Section 4: Effects of too small and too large B/H on model quality [1 mark]
• Section 5: Practical synthesis / Philippine context / NAMRIA standards [1 mark]

Scoring Breakdown

Common Mark Deductions

• Describing the three parameters independently without explaining their interrelationship
• Omitting the formula for air-base in terms of overlap
• Not discussing the consequences of extreme B/H values on model quality
• Writing a general answer without quantitative values (the exam expects numbers like 60%, B/H ≈ 0.6)

Key Phrases To Include

• 60% forward overlap
• air-base B = (1−p) × L
• base-height ratio
• parallax differences
• height precision
• vertical exaggeration
• dead ground
• B/H ≈ 0.6

Marks

2

Topic

Orthophoto / Orthomosaic

Difficulty

medium

Template Id

T15

Examiner Tip

Whenever a question asks for a 'legal or regulatory use,' cite at least one Philippine law (RA, PD, CA, EO) or agency (NAMRIA, LRA, DENR). This immediately signals to the examiner that you have professional-level knowledge.

Model Answer

An orthomosaic is a seamless, georeferenced image map produced by mosaicking (stitching together) multiple individual orthophotos of adjacent areas into a single, continuous, uniform-scale image covering a larger ground area. Production: Individual raw aerial photographs are first orthorectified using a DEM to remove relief displacement and tilt, then colour-balanced and blended at the seam lines between adjacent orthophotos to produce a seamless mosaic. Philippine legal/regulatory use: Under PD 1529 (Property Registration Decree) and the Land Registration Authority (LRA) framework, orthomosaics at large scales (e.g., 1:2,000 or 1:5,000) serve as base imagery for cadastral mapping, the adjudication of land titles, and the delineation of public and private land boundaries — supporting the land titling mandates of RA 8560 (the Geodetic Engineering Act) and the operations of NAMRIA for national base map production.

Question Type

short_answer

Answer Structure

• Definition of orthomosaic (stitched/mosaicked orthophotos, seamless, georeferenced) [1 mark]
• One valid Philippine legal/regulatory use with reference to a law or agency [1 mark]

Scoring Breakdown

Common Mark Deductions

• Defining orthomosaic as simply 'many photos joined together' without specifying that each component is orthorectified
• Giving a general use ('for mapping') without naming a Philippine law, agency, or specific application
• Confusing orthomosaic with a photo mosaic (uncontrolled mosaic) which has no relief correction

Key Phrases To Include

• mosaicking
• seamless
• georeferenced
• orthorectified
• uniform scale
• PD 1529
• RA 8560
• cadastral mapping
• NAMRIA