GELE Photogrammetry & Cartography — Remote Sensing and GISRevision Notes
Final-week revision notes for Remote Sensing and GIS. If you have already studied the full chapter, this page is your go-to refresher before sitting the GELE. Compact, high-yield, and aligned with what Professional Regulation Commission (PRC) — Board of Geodetic Engineering tests in the Photogrammetry & Cartography subtest.
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. Remote Sensing and GIS lands at position 6th 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.
Remote Sensing and GIS - Revision Notes
Remote Sensing (RS) and Geographic Information Systems (GIS) are indispensable tools in modern geodetic engineering practice. Remote sensing acquires spatial information about the Earth's surface without physical contact, using electromagnetic energy recorded by sensors aboard satellites or aircraft. GIS integrates, stores, analyzes, and visualizes spatial and attribute data referenced to a coordinate system. Together, RS and GIS support cadastral surveys, land use mapping, disaster risk reduction, and infrastructure planning in the Philippines — all areas tested in the PRC Geodetic Engineer Licensure Examination. This chapter consolidates all exam-critical concepts, formulas, and worked examples for efficient board review.
Sections
Formulas
Example
Landsat 8 OLI: W = 185 000 m, r = 30 m → n = 185 000 / 30 ≈ 6 167 pixels per line.
Formula
n = W / r
Variables
n = number of pixels across one scan line (dimensionless); W = swath width (m); r = spatial resolution / pixel ground size (m)
Application
Determines how many pixels cover the sensor's across-track swath — used to assess image dimensions and data volume.
Example
UAV at H = 150 m, p = 0.0024 m (2.4 mm pixel pitch), f = 0.024 m (24 mm lens): GSD = (150 × 0.0024) / 0.024 = 0.36 / 0.024 = 15 m — actually GSD = 0.015 m = 1.5 cm.
Formula
GSD = (H × p) / f
Variables
GSD = Ground Sampling Distance / spatial resolution (m); H = sensor altitude above terrain (m); p = detector pitch / pixel size on sensor (m); f = focal length (m)
Application
Relates sensor design parameters to the achievable ground pixel size — fundamental for UAV and aerial photogrammetry planning.
Example
A dense rice paddy: NIR = 0.45, Red = 0.08 → NDVI = (0.45 − 0.08)/(0.45 + 0.08) = 0.37/0.53 ≈ 0.70 (healthy, dense vegetation).
Formula
NDVI = (NIR − Red) / (NIR + Red)
Variables
NDVI = Normalized Difference Vegetation Index (range −1 to +1); NIR = reflectance in the near-infrared band; Red = reflectance in the red band
Application
Quantifies vegetation density and health. Used in Philippine agricultural mapping, reforestation monitoring under DENR programs, and disaster assessment.
Example
Sentinel-2 tile: 10 980 rows × 10 980 cols × 13 bands × 2 bytes (16-bit) ≈ 3.14 GB uncompressed.
Formula
Data volume = rows × cols × bands × (bit depth / 8) bytes
Variables
rows = number of scan lines; cols = pixels per line; bands = number of spectral bands; bit depth = radiometric resolution (bits per pixel)
Application
Estimates raw image file size for storage and transmission planning.
Exam Tips
- Memorize the four resolution types as S-S-R-T (Spatial, Spectral, Radiometric, Temporal) — each can appear as a standalone MCQ or as a comparison question.
- Board exams often pair sensor selection with application context: cloudy/night conditions → SAR (active); daytime crop mapping → multispectral passive optical.
- NDVI formula is frequently tested in computation form — know the range (−1 to +1) and what values indicate: <0 = water/snow, 0–0.2 = bare soil/rock, 0.2–0.5 = sparse vegetation, >0.5 = dense vegetation.
- Swath pixel count (n = W/r) is a straightforward formula computation that regularly appears — practice unit conversions (km to m).
- Know at least two examples per sensor type: passive multispectral (Landsat, Sentinel-2), passive thermal (TIRS on Landsat 8), active radar (Sentinel-1 SAR, ALOS PALSAR), active LiDAR (airborne LIDAR Terrain Mapping — NAMRIA uses this for the Phil LiDAR program).
Key Points
- Remote sensing records reflected or emitted electromagnetic (EM) energy from the Earth's surface using airborne or spaceborne sensors.
- The electromagnetic spectrum relevant to remote sensing spans visible light (0.4–0.7 µm), near-infrared (NIR, 0.7–1.3 µm), short-wave infrared (SWIR, 1.3–2.5 µm), thermal infrared (TIR, 8–14 µm), and microwave (1 mm–1 m).
- Each material (vegetation, water, soil, urban) has a unique spectral signature — the pattern of reflectance or emittance across bands — enabling land-cover discrimination.
- Passive sensors rely on external energy sources (sun for reflected bands, Earth's own heat for thermal). Examples: Landsat OLI/TIRS, Sentinel-2 MSI.
- Active sensors supply their own energy. Examples: Synthetic Aperture Radar (SAR — Sentinel-1, ALOS-2 PALSAR), LiDAR (Light Detection and Ranging).
- Active sensors operate day and night and penetrate cloud cover (microwave radar), making them critical for the frequently cloud-covered Philippine archipelago.
- Image processing workflow: raw data acquisition → geometric correction (rectification/orthorectification) → radiometric correction → image enhancement → classification → accuracy assessment (ground truth).
- Unsupervised classification (e.g., k-means) groups pixels by spectral similarity without training data; supervised classification (e.g., maximum likelihood, SVM, Random Forest) uses analyst-defined training samples.
- Accuracy assessment uses a confusion matrix (error matrix); overall accuracy and Kappa coefficient (κ) are reported.
Definitions
Term
Spatial Resolution
Definition
The smallest ground area represented by one pixel (Ground Sampling Distance, GSD), typically expressed in metres. Example: Landsat 30 m, Sentinel-2 10 m, WorldView-3 0.31 m.
Importance
Determines the minimum feature size detectable — a board exam favourite for distinguishing sensor capabilities.
Term
Spectral Resolution
Definition
The number and width of spectral bands a sensor records. Hyperspectral sensors record hundreds of narrow bands (e.g., AVIRIS: 224 bands, ~10 nm width); multispectral sensors record a few broad bands (e.g., Landsat 8: 11 bands).
Importance
Higher spectral resolution enables finer material discrimination — critical in mineral mapping and vegetation type identification.
Term
Radiometric Resolution
Definition
The number of discrete brightness levels (Digital Numbers, DN) a sensor can record, expressed as bit depth (e.g., 8-bit = 256 levels, 12-bit = 4 096 levels, 16-bit = 65 536 levels).
Importance
Higher bit depth captures subtler tonal differences — important for accurate reflectance retrieval and change detection.
Term
Temporal Resolution
Definition
The frequency with which a sensor revisits and images the same area (revisit time). Example: Landsat 8/9 = 16 days; Sentinel-2 A+B = 5 days at equator; Planet Dove = daily.
Importance
Critical for monitoring dynamic phenomena: typhoon damage, flood extent, crop growth cycles — highly relevant to Philippine disaster management.
Term
Spectral Signature
Definition
The characteristic pattern of a material's reflectance (or emittance) plotted against wavelength. Each land-cover type has a unique signature used for classification.
Importance
The conceptual basis for all remote sensing classification — understanding why vegetation shows high NIR and low Red reflectance (red-edge effect) is exam-essential.
Term
Instantaneous Field of View (IFOV)
Definition
The solid angle through which a detector is sensitive at any instant, expressed in milliradians (mrad). Ground area sampled = IFOV (rad) × H.
Importance
Links sensor optics to spatial resolution — tested in sensor design questions.
Term
Orthorectification
Definition
Geometric correction that removes displacement caused by terrain relief and sensor tilt, producing an orthophoto with uniform map scale referenced to a datum (PRS92 in the Philippines).
Importance
All cadastral and topographic mapping products used in Philippine surveys must be orthorectified and referenced to PRS92/PPCS/UTM.
Section Title
1. Remote Sensing Fundamentals
Common Mistakes
- Confusing spatial resolution with spectral resolution — a sensor can have fine spatial resolution (1 m pixels) but coarse spectral resolution (only 3 bands), or vice versa.
- Calling LiDAR a 'passive' sensor — LiDAR is active (emits laser pulses). Only optical/thermal sensors relying on sunlight or Earth's thermal emission are passive.
- Assuming radar/SAR imagery is always cloud-free — SAR penetrates clouds, but very heavy precipitation can still attenuate C-band signals.
- Forgetting that NDVI uses reflectance values, not raw DN — you must apply radiometric calibration before computing vegetation indices.
- Equating 'higher spatial resolution' with 'better for all tasks' — coarser-resolution sensors (Landsat, MODIS) are preferred for regional/national coverage and time-series analysis; fine-resolution data are costly and data-intensive.
- Ignoring ground truth in classification — accuracy assessment against independent ground-truth points is mandatory; omitting it is scientifically invalid.
Formulas
Example
A 50 m buffer around a borehole: area = π × 50² ≈ 7 854 m² ≈ 0.785 ha.
Formula
Buffer_area = π × d² (circular buffer around a point, radius = d)
Variables
d = buffer distance (m); area in m² or ha (1 ha = 10 000 m²)
Application
Determines the zone of influence around a feature — used in setback analysis (e.g., 3 m easement along cadastral boundary per PD 1529, or 20 m easement along river banks per Water Code).
Example
DEM cell size = 10 m; elevation change between adjacent cells = 5 m → Slope = arctan(5/10) = arctan(0.5) ≈ 26.6°.
Formula
Slope (°) = arctan(Δz / Δx)
Variables
Δz = elevation difference between adjacent cells (m); Δx = horizontal distance between cell centres = raster cell size (m)
Application
Derived from a DEM in GIS for terrain analysis, erosion risk mapping, and road/infrastructure siting.
Example
Two rain gauges: z₁ = 120 mm at d₁ = 5 km; z₂ = 80 mm at d₂ = 10 km; p = 2 → ẑ = (120/25 + 80/100)/(1/25 + 1/100) = (4.8 + 0.8)/(0.04 + 0.01) = 5.6/0.05 = 112 mm.
Formula
IDW estimate: ẑ(x₀) = Σ [z(xᵢ) / dᵢᵖ] / Σ [1 / dᵢᵖ]
Variables
ẑ(x₀) = estimated value at unknown point; z(xᵢ) = known value at sample point i; dᵢ = distance from sample point i to unknown point; p = power parameter (commonly p = 2)
Application
Inverse Distance Weighting interpolation — creates continuous surfaces (rainfall, elevation, pollution) from discrete sample points.
Example
A 1:10 000 scale raster covering one UTM tile at 1 m resolution: 10 000 × 10 000 cells × 4 bytes = 400 MB uncompressed.
Formula
Raster file size = rows × cols × cell_depth (bytes)
Variables
rows, cols = raster dimensions; cell_depth = bytes per cell (e.g., 4 bytes for 32-bit float DEM)
Application
Storage planning for national DEM datasets (NAMRIA, Phil-LiDAR).
Exam Tips
- The PRC board frequently tests vector vs raster applicability — memorise the rule: discrete features (roads, parcels, boundaries) → vector; continuous surfaces (elevation, rainfall, temperature) → raster.
- Know the PPCS zone assignments for major Philippine cities: Metro Manila is in Zone III (PRC exam context); understand that UTM false easting = 500 000 m, central meridian scale factor k₀ = 0.9996.
- For overlay MCQs: 'find areas meeting multiple conditions' = Intersect; 'combine all features' = Union; 'cut one layer with another' = Clip.
- IDW interpolation computation can appear — practice the formula with p = 2 and two or three sample points.
- When a question asks about land titling, registration, or cadastral survey legal authority — link to PD 1529 (Property Registration Decree) for private land and CA 141 (Public Land Act) for public/alienable disposable lands.
- Slope computation from DEM values is a short-answer favourite — apply arctan(Δz/Δx) and convert to degrees.
Key Points
- A GIS links spatial location (where) to attribute data (what) within a database, enabling spatial query and analysis.
- Two fundamental data models: Vector (represents discrete geographic features) and Raster (represents continuous spatial phenomena as a grid of cells).
- Vector entities: Point (0D — e.g., survey monument, BM), Line/Polyline (1D — e.g., road centerline, river), Polygon (2D — e.g., cadastral parcel, municipal boundary).
- Raster cells (pixels) each hold one or more attribute values. Cell size = spatial resolution of the raster. Example: a 10 m DEM means each cell represents a 10 m × 10 m ground area.
- Topology in vector GIS enforces spatial relationships (adjacency, connectivity, containment) — critical for error-free cadastral databases under PD 1529 (Property Registration Decree).
- Common GIS coordinate reference systems in the Philippines: PRS92 (Philippine Reference System 1992) as the national geodetic datum; PPCS/UTM (Philippine Plane Coordinate System, Transverse Mercator, zones 1–5) for large-scale mapping; WGS84 for GPS and global data exchange.
- All GIS layers in a project must share a common datum and projection — mismatched CRS causes layer misalignment, a major source of cadastral errors.
- Key GIS analyses: Overlay (Union, Intersect, Clip), Buffer, Network Analysis (shortest path, service area), Interpolation (IDW, Kriging), Slope/Aspect from DEM.
- Attribute queries use SQL-style expressions; spatial queries use geometric relationships (within, intersects, contains).
- In the Philippine context, GIS supports land administration (PD 1529, CA 141 — Public Land Act), forest boundary delineation, CLUP (Comprehensive Land Use Planning), and disaster risk reduction under DRRM Act (RA 10121).
Definitions
Term
Vector Data Model
Definition
Represents geographic features as geometrically precise points, lines, and polygons with associated attribute tables. Stored in formats such as Shapefile (.shp), GeoJSON, GeoPackage, or ESRI Geodatabase.
Importance
The standard model for cadastral parcels, road networks, and administrative boundaries — central to land administration under PD 1529 and CA 141.
Term
Raster Data Model
Definition
Represents spatial phenomena as a regular grid (matrix) of equally sized cells, each holding a value. Used for DEMs, satellite imagery, land cover grids, and interpolated surfaces.
Importance
Essential for terrain analysis, satellite image processing, and continuous-surface phenomena — the bridge between remote sensing and GIS.
Term
Topology
Definition
Mathematical rules enforcing spatial relationships among vector features (e.g., polygons must not overlap, lines must connect at nodes). Ensures data integrity in cadastral and network datasets.
Importance
Topologically correct parcel maps are required for titling under PD 1529; topology errors can invalidate a cadastral survey.
Term
Overlay Analysis
Definition
GIS operation combining two or more spatial layers to produce a new layer. Types: Union (all features from both layers), Intersect (common area only), Clip (cookie-cutter). Requires identical coordinate systems.
Importance
Core analytical operation — regularly tested as a scenario-based MCQ (e.g., 'find agricultural land within a flood-prone area').
Term
Digital Elevation Model (DEM)
Definition
A raster representation of terrain elevation values. Variants: DTM (Digital Terrain Model, bare earth after removing vegetation/structures), DSM (Digital Surface Model, includes all surface objects). Derived from LiDAR, InSAR, stereo photogrammetry, or contour interpolation.
Importance
Fundamental GIS layer for slope, watershed, viewshed, and flood analyses — and a key output of LiDAR surveys conducted by NAMRIA under the Phil-LiDAR 1 and 2 programs.
Term
Coordinate Reference System (CRS)
Definition
The framework defining how coordinates relate to real positions on Earth, comprising a datum (geodetic or vertical) and a map projection. In the Philippines: PRS92 datum + PPCS/UTM projection for national mapping; WGS84 for GPS.
Importance
Layer mismatch due to wrong CRS is the single most common GIS error in practice and a favourite board exam pitfall scenario.
Term
Attribute Table
Definition
A database table linked to a vector layer where each row corresponds to one feature and each column to one attribute (e.g., parcel ID, area in m², owner name, land use classification).
Importance
The 'what' side of GIS — spatial queries and thematic maps are generated by querying attribute tables.
Section Title
2. GIS Data Models and Core Analysis
Common Mistakes
- Using raster for discrete cadastral parcels — parcel boundaries are precise legal lines; raster cells cannot represent exact boundary positions. Always use vector polygons for cadastral data.
- Overlaying layers in different coordinate systems without re-projecting — this is the leading cause of spatial misalignment in GIS projects. Always verify CRS before overlay.
- Confusing DEM, DTM, and DSM — DTM is bare earth (vegetation removed); DSM includes all surface objects; DEM is a generic term but often used interchangeably with DTM in Philippine practice.
- Treating raster cell size as spatial resolution when it has been resampled — resampling to a finer cell size does not improve actual spatial resolution; it only changes the grid size.
- Forgetting that GIS buffer distances must match the projection units — in geographic coordinates (degrees), buffer distances in metres are incorrect; always project to a metric CRS (PPCS/UTM) before buffering.
Formulas
Example
A point in Zone III (λ₀ = 121°E): if the simplified transverse Mercator offset gives 25 000 m east of the central meridian, then E = 500 000 + 0.9996 × 25 000 ≈ 524 990 m.
Formula
PPCS Easting: E = 500 000 + k₀ × N × (λ − λ₀)
Variables
E = Easting (m); k₀ = 0.9996 (UTM scale factor); N = radius of curvature in prime vertical; λ = longitude of point; λ₀ = central meridian of the zone
Application
Converts geodetic coordinates (φ, λ) on PRS92 to PPCS/UTM plane coordinates — used in all cadastral and topographic survey computations in the Philippines.
Example
At E = 600 000 m (100 km east of CM): k ≈ 0.9996 × (1 + 100 000² / (2 × 6 371 000²)) ≈ 0.9996 × 1.00123 ≈ 1.0008.
Formula
Area scale factor: k = k₀ × (1 + (E − 500 000)² / (2 × R² × k₀²))
Variables
k = local scale factor; k₀ = 0.9996; E = easting (m); R = mean radius of Earth ≈ 6 371 000 m
Application
Corrects map distances and areas for the distortion inherent in the Transverse Mercator projection — critical for precise cadastral area computations.
Exam Tips
- Always link sensor/method choice to Philippine geographic realities: typhoons, cloud cover, island geography, and tropical vegetation — these contextual factors often appear in board exam scenario questions.
- RA 4374 scope: geodetic engineering includes horizontal and vertical control surveys, cadastral surveys, photogrammetric surveys, hydrographic surveys, and RS mapping — know all five.
- PD 1529 (Property Registration Decree) governs the Torrens system of land registration — any question about land titling, OCT, TCT, or annotation references this law.
- CA 141 (Commonwealth Act 141, Public Land Act) governs the classification, administration, and disposition of alienable and disposable (A&D) lands — GIS is used to delineate A&D vs forest land boundaries.
- For any computation involving Philippine cadastral coordinates, confirm the zone (I–V) and use k₀ = 0.9996 unless stated otherwise.
Key Points
- The National Mapping and Resource Information Authority (NAMRIA) is the central mapping agency of the Philippines, producing topographic maps, nautical charts, and geodetic control using RS and GIS.
- Phil-LiDAR 1 (DREAM) and Phil-LiDAR 2 programs produced nationwide airborne LiDAR-derived DEM/DTM data used for flood hazard mapping by PHIVOLCS, MGB, and DOST-PAGASA.
- Sentinel-1 SAR data (free, ESA Copernicus) is operationally used for flood inundation mapping during Philippine typhoon events — a direct application of active RS in disaster response.
- The PPCS (Philippine Plane Coordinate System) uses the Transverse Mercator projection in five zones (Zones I–V), referenced to PRS92 (GRS80 ellipsoid, ITRF92 realization at epoch 1991.0).
- RA 8560 (PRC Modernization Act) and RA 4374 (Geodetic Engineering Act) define the scope of geodetic engineering practice, which explicitly includes photogrammetric and remote sensing surveys.
- Cadastral surveys under PD 1529 must be tied to the national geodetic network (PRS92 control points established by NAMRIA); GIS parcel databases must be georeferenced to PPCS/UTM.
- Change detection in GIS/RS: comparing multi-temporal images or datasets to identify land-cover change (e.g., deforestation monitoring, urban expansion, coastal erosion along Philippine coastlines).
- WebGIS and geoportals (e.g., NAMRIA GeoPortal, PHIVOLCS FASSSTER) make spatial data publicly accessible for planning agencies.
- Open-source GIS tools (QGIS, GRASS GIS) and commercial platforms (ArcGIS, ERDAS IMAGINE) are used in Philippine government and private geodetic practice.
Definitions
Term
PRS92 (Philippine Reference System 1992)
Definition
The national geodetic datum of the Philippines, based on the GRS80 ellipsoid, realized through the ITRF92 frame at epoch 1991.0. Replaces the older Luzon Datum (Clarke 1866 ellipsoid).
Importance
All legally valid geodetic surveys in the Philippines must be referenced to PRS92 — directly tested in board exams in both RS/GIS and geodesy subjects.
Term
PPCS (Philippine Plane Coordinate System)
Definition
Five-zone Transverse Mercator grid system applied over PRS92, covering Philippine territory. Zones I–V run from Luzon (Zone I) to Mindanao/Sulu (Zone V). Scale factor k₀ = 0.9996, false easting = 500 000 m, false northing = 0 m.
Importance
The standard plane coordinate system for all cadastral and topographic mapping — a required reference in any GIS project involving Philippine spatial data.
Term
Change Detection
Definition
RS/GIS process of comparing spatially co-registered, multi-temporal datasets (images or maps) to identify and quantify changes in land cover, land use, or physical features over time.
Importance
Used in monitoring deforestation (under RA 7586 — NIPAS Act), urban sprawl, and coastal erosion — a common application scenario in board exam questions.
Term
NAMRIA
Definition
National Mapping and Resource Information Authority — the central geodetic surveying and mapping authority of the Philippines under DND, responsible for topographic, cadastral, and nautical mapping, as well as maintaining national geodetic control networks.
Importance
Frequently referenced in board exam questions on agency jurisdiction, map standards, and geodetic datum maintenance.
Section Title
3. Integration of RS and GIS in Philippine Geodetic Practice
Common Mistakes
- Using WGS84 coordinates directly in PPCS/UTM computations without noting the very small but practically significant difference from PRS92 (~1–2 m) — for high-precision cadastral surveys, this offset matters.
- Assigning a point in Metro Manila to the wrong PPCS zone — Metro Manila falls in Zone III (central meridian 121°E); Palawan is Zone I (117°E); Mindanao is mainly Zone IV (123°E) and Zone V (125°E).
- Confusing RA 4374 and RA 8560 — RA 4374 is the Geodetic Engineering Act of 1965 (defines the profession); RA 8560 is the PRC Modernization Act of 1998 (governs the PRC and licensure examinations).
- Treating NAMRIA topographic maps (1:50 000 scale) as cadastral maps — topographic maps are for navigation and resource mapping, not parcel boundary definition; cadastral maps are produced under PD 1529 authority.
Connections
- Remote sensing image rectification and orthorectification directly apply photogrammetric principles (Chapter: Photogrammetry Fundamentals) — both use ground control points (GCPs) on PRS92 for georeferencing.
- PPCS/UTM coordinates used in GIS are the output of geodetic control surveys (Chapter: Geodetic Control Networks) — GIS is the end-user platform for survey data.
- DEM generation from LiDAR or stereo photogrammetry connects RS to Topographic Surveying — slope, aspect, contour, and watershed analyses are GIS extensions of traditional terrain work.
- Cadastral GIS databases link directly to land law (PD 1529, CA 141) and land surveying (Chapter: Cadastral Surveying) — parcel geometry is the spatial backbone of the Torrens title system.
- Spectral signatures and band ratios (e.g., NDVI) connect to physics of the electromagnetic spectrum — understanding why vegetation absorbs Red and reflects NIR requires knowledge of plant biophysics.
- Coordinate transformations between WGS84 (GPS) and PRS92 (national datum) — covered in geodesy — are essential operations performed every time GPS survey data is imported into a Philippine GIS project.
- Accuracy assessment of RS classifications uses statistical concepts (confusion matrix, Kappa coefficient) — connects to the statistics and error theory module in the geodetic engineering curriculum.
- Temporal resolution and change detection connect to environmental laws: RA 7586 (NIPAS), RA 9003 (Ecological Solid Waste Management), RA 9729 (Climate Change Act) — all require periodic spatial monitoring using RS/GIS.
Exam Strategy
For the RS and GIS portion of the PRC Geodetic Engineer board exam, prioritise three skill clusters: (1) CONCEPT IDENTIFICATION — be able to classify sensor types (active/passive), resolution types (S-S-R-T), and data models (vector/raster) from a brief scenario description; these appear frequently as MCQs. (2) FORMULA COMPUTATION — master four calculations: pixel count (n = W/r), NDVI, IDW interpolation (2-point case), and slope from DEM; each can yield a 5–10 point problem. (3) LEGAL AND INSTITUTIONAL CONTEXT — link every spatial dataset or survey activity to the correct Philippine law or agency: NAMRIA for mapping and control, PD 1529 for cadastral/titling, CA 141 for public lands, RA 4374 for professional scope. In objective-type questions, eliminate options that confuse passive for active sensors or vector for raster data models — these are the two highest-frequency distractors. For scenario questions, always state your assumption about the operating environment (cloud cover, time of day, feature type) before selecting the sensor or method — this structured reasoning earns full marks in written/essay portions. Allocate 2–3 minutes per RS/GIS computation MCQ and flag any question involving PPCS zone determination for careful geographic reasoning.
Quick Review Questions
A satellite sensor has a spatial resolution of 10 m and a swath width of 290 km. How many pixels span one across-track scan line?
Apply n = W / r: W = 290 000 m, r = 10 m → n = 290 000 / 10 = 29 000 pixels. This is a direct formula application — always convert swath width to metres before dividing.
A disaster response team needs to map flood inundation in Cagayan Valley during Typhoon Ulysses. It is night-time and heavily overcast. Which sensor type should they use, and why?
SAR sensors supply their own microwave energy, penetrate clouds and rain (especially L-band and C-band), and operate day and night. Passive optical sensors require sunlight and clear skies — neither available in this scenario. Operational examples: Sentinel-1 (C-band SAR, free ESA data) or ALOS-2 PALSAR-2 (L-band, better penetration through dense vegetation).
Classify the following as vector or raster: (a) road centerlines, (b) rainfall distribution surface, (c) cadastral parcel boundaries, (d) slope map derived from a DEM.
Discrete features with precise geometry (roads, parcel boundaries) → vector. Continuous surfaces representing a value that varies smoothly across space (rainfall, slope) → raster. Note: slope is derived from a DEM (raster) and is itself a raster grid.
Calculate NDVI for a pixel with NIR reflectance = 0.52 and Red reflectance = 0.06. Interpret the result.
NDVI = (NIR − Red)/(NIR + Red) = (0.52 − 0.06)/(0.52 + 0.06) = 0.46/0.58 ≈ 0.793. Values above 0.5 indicate dense green vegetation (e.g., tropical forest or irrigated cropland). NDVI range: −1 (open water) to +1 (dense vegetation).
Why must all GIS layers in a cadastral project share the same coordinate reference system?
Each CRS defines a unique mathematical relationship between Earth's surface and the plane. A layer in PRS92/PPCS Zone III and another in WGS84/Geographic will appear offset by 1–2 m or more. For cadastral surveys under PD 1529, all data must be in PRS92 + PPCS/UTM to maintain legal boundary integrity.
What is the difference between a DTM and a DSM?
LiDAR point clouds are classified into ground points (used for DTM) and non-ground points (vegetation, structures). The DTM is used for hydrological modelling (flood simulation) because water flows over bare terrain, not over tree canopies.
A point lies 80 km east of the central meridian in PPCS Zone III. Using the scale factor formula k = k₀ × (1 + d²/(2R²)), where d = 80 000 m and R = 6 371 000 m, compute the approximate local scale factor.
k = 0.9996 × (1 + (80 000)²/(2 × (6 371 000)²)) = 0.9996 × (1 + 6.4×10⁹ / 8.118×10¹³) = 0.9996 × (1 + 0.0000788) = 0.9996 × 1.0000788 ≈ 0.9996 + 0.0000789 ≈ 1.0004 (rounded). This means distances at this location are 0.04% longer on the map than on the ground — a small but legally significant correction in cadastral surveys.
Which Philippine law governs the practice of geodetic engineering, and does it include remote sensing and photogrammetric surveys within its scope?
RA 4374 defines geodetic engineering to include: geodetic surveys, cadastral surveys, hydrographic surveys, photogrammetric surveys, and remote sensing mapping. Practicing these without a valid PRC Geodetic Engineer license is illegal. RA 8560 (PRC Modernization Act) governs the licensure examination process.
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