Philippine Plane Coordinate System and UTM - Revision Notes
Field surveys are computed on plane grids because flat-surface mathematics is far simpler than ellipsoidal geodesy. The Philippines operates two Transverse Mercator (TM) plane coordinate systems: the national Philippine Plane Coordinate System (PPCS), defined on the PRS92 datum (Clarke 1866 ellipsoid), and the worldwide Universal Transverse Mercator (UTM), commonly referenced to WGS84. Both systems project the curved Earth onto a flat plane using the Transverse Mercator projection, but they differ in zone width, central meridian (CM) scale factor, and datum. Mastery of these two systems — their zones, false origins, scale factors, and conversion formulas — is consistently tested in the PRC Geodetic Engineer Licensure Examination under Photogrammetry and Cartography.
1. Transverse Mercator Projection — Conceptual Foundation
Formulas
k = k₀ × [1 + (p²/2R²) + …]
Example
At the CM, p = 0, so k = k₀. At the zone edge, p is large and k > k₀.
Variables
k = scale factor at distance p from CM; k₀ = CM scale factor; R = Earth radius (~6,371 km)
Application
Explains why distortion grows away from the central meridian. On board exams, the exact expansion is rarely needed, but understanding the relationship is essential.
Exam tips
- Memorise: UTM k₀ = 0.9996 (4 nines after decimal = 0.9996); PPCS k₀ = 0.99995 (5 nines = closer to 1 because zones are narrower).
- The secant TM is always the one used in practice — tangent TM is only conceptual.
- False easting = 500,000 m is a universal constant for both systems in the Philippines.
Key points
- The Transverse Mercator (TM) projection wraps a cylinder tangent (or secant) to the ellipsoid along a meridian called the central meridian (CM).
- The CM is the north–south axis of the grid zone; it is where distortion is minimum (scale factor equals k₀ exactly).
- Away from the CM, the scale factor increases, causing length distortion — this is why zone widths are kept narrow.
- Setting k₀ slightly below 1.0 (secant case) reduces the zone's overall distortion: scale is <1 near the CM, =1 at two parallel lines flanking the CM, and >1 at the zone edges.
- A false easting of 500,000 m is added so that all eastings within a zone are positive (the CM is given a grid easting of 500,000 m).
- A false northing of 0 m (N hemisphere) or 10,000,000 m (S hemisphere) keeps northings positive.
- The Philippines lies entirely in the Northern Hemisphere, so false northing = 0 for both PPCS and UTM (northern zones).
Definitions
Central Meridian (CM)
The meridian of longitude that passes through the centre of a TM zone. It defines the origin of grid eastings (before adding the false easting).
Importance
Knowing the CM of a zone lets you compute the true distance of a point from the CM and thus its grid easting.
False Easting
A constant value (500,000 m in both PPCS and UTM) added to all computed eastings so that no grid easting is negative within the zone.
Importance
Essential for computing grid coordinates from field data. Forgetting the false easting is one of the most common board exam errors.
Scale Factor (k₀)
The ratio of a grid distance to the corresponding ellipsoidal geodesic distance at the CM. Values: UTM k₀ = 0.9996; PPCS k₀ = 0.99995.
Importance
Used to convert between ground distances and grid distances: Grid Dist = Ground Dist × k₀.
Common mistakes
- Confusing the CM scale factor of UTM (0.9996) with that of PPCS (0.99995) — they differ by a factor of ~5.
- Assuming the Philippines uses UTM exclusively — national surveys use PPCS on PRS92, while GPS data often arrives in UTM/WGS84.
- Forgetting that the false easting is always 500,000 m for both systems.
2. Universal Transverse Mercator (UTM)
Formulas
Zone = ⌊(λ + 180) / 6⌋ + 1
Example
λ = 121°E: Zone = ⌊(121 + 180)/6⌋ + 1 = ⌊301/6⌋ + 1 = ⌊50.167⌋ + 1 = 50 + 1 = 51 → Zone 51N ✓
Variables
Zone = UTM zone number (integer 1–60); λ = longitude in decimal degrees (east = positive, west = negative); ⌊ ⌋ = floor function (round down to nearest integer)
Application
Converts any longitude to its UTM zone number. This is the most frequently tested formula for this topic.
CM = 6 × Zone − 183 (degrees)
Example
Zone 51: CM = 6(51) − 183 = 306 − 183 = 123°E ✓
Variables
CM = longitude of central meridian in degrees east; Zone = UTM zone number
Application
Find the central meridian of any UTM zone. Useful for computing distance of a point from its CM.
E_grid = 500,000 + d_CM
Example
Point 3,000 m west of CM: E = 500,000 + (−3,000) = 497,000 m
Variables
E_grid = grid easting in metres; d_CM = signed distance from CM in metres (positive = east of CM, negative = west of CM)
Application
Compute the grid easting of any point given its distance from the zone CM.
Exam tips
- The UTM zone formula ⌊(λ+180)/6⌋+1 must be memorised exactly — it appears in nearly every board exam set on this topic.
- For any Philippine longitude between 120°E and 126°E, the answer is Zone 51N. For longitudes between 114°E and 120°E, the answer is Zone 50N.
- CM formula check: Zone 50 → CM = 6(50)−183 = 117°E; Zone 51 → CM = 6(51)−183 = 123°E. Memorise these for quick answers.
- If the question gives a point on the boundary meridian (e.g., exactly 120°E), apply the formula: ⌊(120+180)/6⌋+1 = ⌊50⌋+1 = 51 → Zone 51N (boundary belongs to the higher zone).
Key points
- UTM divides the globe into 60 zones, each 6° of longitude wide, numbered 1 to 60 eastward starting from the 180° meridian.
- Zone 1: 180°W to 174°W; Zone 2: 174°W to 168°W; … each zone is 6° wide.
- The CM of Zone N is at longitude: CM = (6N − 183)°.
- Philippines longitude range: approximately 116°E to 127°E → falls in UTM Zones 50 and 51 (Northern Hemisphere).
- Zone 50N: CM = 117°E (covers 114°E–120°E); Zone 51N: CM = 123°E (covers 120°E–126°E).
- UTM uses WGS84 datum for global GPS applications.
- UTM parameters: k₀ = 0.9996; False Easting = 500,000 m; False Northing = 0 (N hemisphere).
- Grid coordinate designation: e.g., '51N 123456E 1234567N' specifies zone, easting, northing.
Definitions
UTM Zone
One of 60 longitudinal strips, each 6° wide, within which the Transverse Mercator projection is applied with a unique central meridian.
Importance
Specifying the correct zone is mandatory for valid UTM coordinates; coordinates in adjacent zones are computed in different grids.
Floor Function ⌊x⌋
Returns the largest integer less than or equal to x. E.g., ⌊50.167⌋ = 50, ⌊49.999⌋ = 49.
Importance
Critical for correctly applying the UTM zone formula. A common error is rounding instead of flooring.
WGS84
World Geodetic System 1984 — the datum used by GPS and UTM globally. Ellipsoid parameters: a = 6,378,137.0 m; f = 1/298.2572.
Importance
Understanding the datum difference (WGS84 vs PRS92) is key when converting between UTM and PPCS coordinates.
Common mistakes
- Rounding (λ+180)/6 instead of applying the floor function — always truncate down.
- Using the wrong sign for λ (west longitudes are negative in the formula).
- Stating the Philippines is in Zone 50 only — it spans both Zones 50 and 51.
- Forgetting to add 'N' or 'S' to the zone designation when specifying hemisphere.
3. Philippine Plane Coordinate System (PPCS)
Formulas
PPCS Zone = nearest odd zone such that |λ − CM| is minimised, where CM ∈ {117°, 119°, 121°, 123°, 125°}
Example
λ = 120.5°E: Distances to CMs — |120.5−119|=1.5°, |120.5−121|=0.5° → nearest CM = 121°E → Zone III
Variables
λ = longitude of the point in degrees east; CM = central meridian of candidate PPCS zone
Application
Assign a point to its PPCS zone by finding which CM is closest to the point's longitude.
E_PPCS = 500,000 + (λ − CM_rad) × N × cos φ
Example
A point 2,300 m east of CM 121°E: E = 500,000 + 2,300 = 502,300 m
Variables
Simplified form; exact computation requires TM series expansion. For board purposes: E = 500,000 + d_CM (m)
Application
Conceptual: grid easting = false easting + departure from CM. Exact TM computation uses geodetic latitude φ, longitude λ, and ellipsoid parameters.
Exam tips
- Memorise the PPCS CM table as a sequence: 117, 119, 121, 123, 125 (zones I–V, every 2°).
- Quick zone check: Zone II covers Palawan and western Visayas; Zone III covers Luzon; Zone IV covers eastern Visayas and Mindanao; Zone V covers eastern Mindanao and Samar.
- The CM spacing (2°) of PPCS versus zone width (6°) of UTM is a favourite exam distinction.
- k₀ comparison mnemonic: 'PPCS is closer to 1 because its zones are narrower' (0.99995 > 0.9996).
Key points
- PPCS was established under Executive Order No. 45 (1936) and modernised with the adoption of PRS92.
- PRS92 (Philippine Reference System of 1992) is based on the Clarke 1866 ellipsoid: a = 6,378,206.4 m; b = 6,356,583.8 m; f = 1/294.9786982.
- PPCS divides the Philippines into 5 zones, each a Transverse Mercator projection with a 2°-wide CM spacing.
- PPCS Zone Table: Zone I → CM 117°E; Zone II → CM 119°E; Zone III → CM 121°E; Zone IV → CM 123°E; Zone V → CM 125°E.
- Each PPCS zone covers approximately ±1° on either side of its CM (about 2° total width).
- PPCS parameters: k₀ = 0.99995; False Easting = 500,000 m; False Northing = 0 m.
- The datum PRS92 is practically equivalent to NAD27 (North American Datum 1927) in terms of the ellipsoid used (Clarke 1866).
- PPCS is used for all cadastral surveys, land titling, and engineering surveys under PD 1529 (Property Registration Decree) and CA 141 (Public Land Act).
- Conversion from WGS84/UTM to PRS92/PPCS requires a 7-parameter Helmert transformation.
Definitions
PRS92
Philippine Reference System of 1992. The official geodetic datum of the Philippines, based on the Clarke 1866 ellipsoid, realised through a network of VLBI and GPS-controlled triangulation stations.
Importance
All legal land surveys in the Philippines must be referenced to PRS92/PPCS under NAMRIA regulations.
Clarke 1866 Ellipsoid
The reference ellipsoid for PRS92 with semi-major axis a = 6,378,206.4 m and flattening f = 1/294.9786982. Also the basis of NAD27.
Importance
The datum difference between PRS92 (Clarke 1866) and WGS84 results in coordinate shifts of several metres that must be accounted for in precision surveys.
PPCS Zone III
The PPCS zone with central meridian at 121°E. It covers most of Luzon including Metro Manila, making it the most commonly used PPCS zone in Philippine practice.
Importance
Most government and engineering surveys in Luzon use Zone III. Expect board questions to use Zone III coordinates as examples.
RA 8560
Republic Act 8560 — The Philippine Geodetic Engineering Act of 1998. Defines the professional scope of geodetic engineers, including the conduct of surveys using the official Philippine coordinate systems.
Importance
Legal basis for geodetic engineering practice; establishes that surveys must conform to NAMRIA-prescribed datums and coordinate systems.
Common mistakes
- Confusing PPCS zone numbers with Roman numerals I–V versus UTM zone numbers 50–51.
- Using UTM k₀ = 0.9996 for PPCS computations — PPCS uses k₀ = 0.99995.
- Assuming PPCS is on WGS84 — it is on PRS92 (Clarke 1866).
- Mixing up the 2°-wide PPCS zones with the 6°-wide UTM zones.
- Assigning a point on the boundary between two PPCS zones without checking which CM is closer.
4. Comparing PPCS and UTM — Key Differences
Exam tips
- A classic board question: 'Which system uses k₀ = 0.9996?' Answer: UTM. 'Which uses k₀ = 0.99995?' Answer: PPCS.
- The phrase 'official datum for Philippine surveys' always refers to PRS92/PPCS, never WGS84/UTM.
- Remember: PPCS has MORE zones for a smaller area (5 zones for PH) because it uses narrower 2° zones to reduce distortion.
Key points
- Zone width: PPCS = ~2° (5 zones for PH); UTM = 6° (60 zones worldwide).
- CM scale factor: PPCS k₀ = 0.99995; UTM k₀ = 0.9996.
- Datum: PPCS on PRS92 (Clarke 1866); UTM commonly on WGS84.
- Number of zones covering PH: PPCS = 5 zones (I–V); UTM = 2 zones (50N, 51N).
- Legal status: PPCS/PRS92 is the official national datum for cadastral and engineering surveys; UTM/WGS84 is used for GPS positioning and international applications.
- False easting: Both use 500,000 m. False northing: Both use 0 m for northern hemisphere.
- The narrower PPCS zones produce less distortion (smaller scale correction) than UTM zones.
- Zone I (CM 117°E) aligns exactly with UTM Zone 50 CM (117°E) — but they are on different datums.
Definitions
Datum Transformation
A mathematical conversion (e.g., 7-parameter Helmert/Bursa-Wolf) used to convert coordinates from one datum to another, such as WGS84 to PRS92.
Importance
Necessary whenever GPS data (WGS84) must be incorporated into a PRS92/PPCS project. Errors in datum transformation can cause metre-level positional errors.
Common mistakes
- Treating PPCS and UTM as interchangeable — they are on different datums and have different scale factors.
- Stating that the Philippines has 2 PPCS zones (confusing with UTM zones).
- Assuming Zone I CM (117°E) = Zone 50 CM means the coordinates are the same — they differ because of datum differences.
5. Board-Style Worked Problems
Formulas
Zone_UTM = ⌊(λ + 180) / 6⌋ + 1
Example
PROBLEM: Find the UTM zone for Davao City at λ = 125.61°E. SOLUTION: Zone = ⌊(125.61+180)/6⌋+1 = ⌊305.61/6⌋+1 = ⌊50.935⌋+1 = 50+1 = 51 → Zone 51N
Variables
λ in decimal degrees east
Application
Find UTM zone number for any longitude.
CM_UTM = 6 × Zone − 183
Example
Zone 50: CM = 6(50)−183 = 300−183 = 117°E
Variables
CM in degrees east; Zone = integer zone number
Application
Find the CM of any UTM zone.
E = 500,000 + d
Example
PROBLEM: A triangulation station is 4,500 m east of its PPCS CM. Find its easting. SOLUTION: E = 500,000 + 4,500 = 504,500 m
Variables
E = grid easting (m); d = signed distance from CM in metres (+east, −west)
Application
Compute grid easting from CM departure.
Grid Distance = Ground Distance × k₀
Example
PROBLEM: A line measured 1,000.00 m on the ground. Find its PPCS grid distance. SOLUTION: Grid Dist = 1,000.00 × 0.99995 = 999.95 m
Variables
k₀ = 0.9996 (UTM) or 0.99995 (PPCS); Ground Distance in metres
Application
Convert measured ground distance to grid distance for plotting on the coordinate grid.
Exam tips
- For multiple-choice questions, always compute the UTM zone formula step by step — never guess from memory alone.
- When a problem says 'a point is X metres west of the CM,' the departure d is negative: E = 500,000 − X.
- Scale factor problems: if k₀ < 1, grid distance < ground distance (grid 'shrinks' relative to ground). This is always true for TM projections at the CM.
- Check your answer: any valid Philippine easting should be between ~400,000 m and ~600,000 m for points within ±100 km of the CM.
Key points
- Problem-solving sequence: (1) Identify the system (PPCS or UTM); (2) Apply zone formula or CM table; (3) Apply false easting; (4) State final answer with units and zone designation.
- Always show the floor function step explicitly — examiners check this.
- Grid easting = false easting ± departure from CM (positive if east, negative if west of CM).
- Scale correction: Grid Distance = Ellipsoidal Distance × k₀.
- Conversion sequence for GPS data into cadastral survey: WGS84 → PRS92 datum transformation → PPCS coordinates.
Common mistakes
- Applying scale factor correction in the wrong direction (multiplying when should divide, or vice versa).
- Not specifying 'N' (northern hemisphere) in the UTM zone designation.
- Using 6° zone width logic to assign PPCS zones — PPCS zones are only ~2° wide.
6. Legal and Regulatory Framework
Exam tips
- Board exams routinely ask: 'What law governs geodetic engineering practice in the Philippines?' → RA 8560 (amended RA 4374).
- PD 1529 = land registration → PPCS/PRS92 coordinates required.
- CA 141 = public lands → surveys must use official datum.
- Memorise: NAMRIA = maps + datum maintenance; LRA = land titles + cadastral records.
Key points
- RA 8560 (Philippine Geodetic Engineering Act of 1998): defines geodetic engineering practice; requires all surveys to use the official national datum (PRS92).
- RA 4374 (as amended by RA 8560): original Geodetic Engineering Act; established the profession of geodetic engineering in the Philippines.
- PD 1529 (Property Registration Decree, 1978): requires all land titles and cadastral surveys to be in PPCS/PRS92 coordinates.
- CA 141 (Public Land Act, 1936): governs classification and disposition of public lands; surveys of public lands must conform to official coordinate systems.
- NAMRIA (National Mapping and Resource Information Authority): the official mapping agency that maintains PRS92 control points and publishes transformation parameters between WGS84 and PRS92.
- All cadastral surveys filed with the Land Registration Authority (LRA) must carry PPCS coordinates on PRS92.
- Under RA 8560, only licensed Geodetic Engineers may conduct surveys that establish or re-establish control monuments in the Philippine coordinate system.
Definitions
NAMRIA
National Mapping and Resource Information Authority. The Philippine government agency responsible for maintaining the national geodetic datum, producing topographic maps, and providing nautical charts.
Importance
NAMRIA is the authoritative source for PRS92 control point coordinates and WGS84–PRS92 transformation parameters used in all official surveys.
LRA
Land Registration Authority. The agency that processes and maintains records of land titles in the Philippines under PD 1529.
Importance
All plans submitted to LRA must carry PPCS/PRS92 coordinates — a direct legal requirement linking geodetic engineering to national coordinate systems.
Common mistakes
- Confusing RA 4374 with RA 8560 — RA 8560 is the current, amended law governing geodetic engineering.
- Forgetting that PD 1529 mandates PPCS coordinates on all land title documents.
- Assuming UTM/WGS84 is acceptable for cadastral surveys — only PRS92/PPCS is legally recognised for titling purposes.
Connections
- PPCS and UTM are both applications of the Transverse Mercator projection — understanding TM projection fundamentals (cylinder orientation, secant case, scale factor variation) explains the design choices for both systems.
- The PRS92 datum (Clarke 1866) connects to geodesy fundamentals: ellipsoid parameters, geoid-ellipsoid separation, and datum transformations are prerequisite knowledge for understanding coordinate differences between PPCS and UTM/WGS84.
- Cadastral surveying (a major PRC exam subject) relies directly on PPCS Zone designations and PRS92 coordinates — every plan submitted to LRA or DENR must carry PPCS coordinates as required by PD 1529.
- GPS surveying (another exam topic) delivers WGS84 coordinates, requiring a 7-parameter Helmert transformation to PRS92 before PPCS coordinates can be computed — linking this chapter to datum transformation mathematics.
- Photogrammetric ground control (aerial triangulation) uses PPCS/PRS92 for horizontal control and the Philippine Vertical Datum (mean sea level, Manila Tide Gauge) for vertical — connecting this chapter to aerial survey control.
- Map projection theory (distortion, scale factor, conformal projections) provides the mathematical underpinning for both PPCS and UTM — understanding conformality explains why the TM is preferred over other projections for narrow-zone national grids.
- RA 8560 (Geodetic Engineering Act) ties the technical content of PPCS/UTM directly to the legal scope of practice — board examiners frequently test whether candidates know which law mandates which datum.
Quick review questions
Find the UTM zone for a point at longitude 117°E.
Answer
Zone 50N
Zone = ⌊(117+180)/6⌋+1 = ⌊297/6⌋+1 = ⌊49.5⌋+1 = 49+1 = 50. The Philippines is in the Northern Hemisphere, so the designation is Zone 50N.
Find the UTM zone for a point at longitude 124°E.
Answer
Zone 51N
Zone = ⌊(124+180)/6⌋+1 = ⌊304/6⌋+1 = ⌊50.667⌋+1 = 50+1 = 51 → Zone 51N.
Which PPCS zone covers a point at longitude 118.7°E?
Answer
Zone II (CM = 119°E)
Compute distances to candidate CMs: |118.7−117|=1.7°, |118.7−119|=0.3°. The nearest CM is 119°E → PPCS Zone II.
A point is 4,500 m east of the central meridian. What is its grid easting?
Answer
504,500 m
E = 500,000 + (+4,500) = 504,500 m. East of CM gives a positive departure.
What is the central meridian scale factor for PPCS? For UTM?
Answer
PPCS: k₀ = 0.99995; UTM: k₀ = 0.9996
PPCS zones are narrower (~2°), so less distortion correction is needed, allowing k₀ to be closer to 1.0 than the wider UTM zones (6°).
A ground distance of 2,000.00 m is measured near the CM of a PPCS zone. What is the corresponding grid distance?
Answer
1,999.90 m
Grid Distance = Ground Distance × k₀ = 2,000.00 × 0.99995 = 1,999.90 m. The grid distance is slightly shorter because k₀ < 1 at the CM.
What datum is PRS92 based on, and what is the semi-major axis of its ellipsoid?
Answer
Clarke 1866 ellipsoid; a = 6,378,206.4 m
PRS92 uses the Clarke 1866 ellipsoid, the same as NAD27. This ellipsoid is older and smaller than the WGS84 ellipsoid (a = 6,378,137.0 m), causing coordinate differences of several metres between the two systems.
What Philippine law requires that all land title surveys carry PPCS/PRS92 coordinates?
Answer
Presidential Decree 1529 (Property Registration Decree)
PD 1529 governs the registration of land in the Philippines and requires all cadastral and title surveys to reference the official national coordinate system (PRS92/PPCS), as administered through NAMRIA.
What is the central meridian of UTM Zone 50?
Answer
117°E
CM = 6 × 50 − 183 = 300 − 183 = 117°E. This coincides with the CM of PPCS Zone I, though the two systems use different datums.
A point has a grid easting of 493,500 m. Is it east or west of the CM, and by how much?
Answer
West of CM by 6,500 m
Departure from CM = E − 500,000 = 493,500 − 500,000 = −6,500 m. Negative departure means west of the CM.
Diagrams
Philippine Coordinate Systems — Chapter Overview Mind Map
A comprehensive mind map showing the relationships between UTM and PPCS systems, their parameters, zone structures, legal framework, and key formulas. Use this for a quick chapter overview before study and again for rapid recall during review.
Step-by-Step UTM Zone Determination Procedure
A step-by-step flowchart for computing the UTM zone number from a given longitude. Follows the exact examination procedure: compute (λ+180), divide by 6, apply floor, add 1, then append hemisphere letter. Includes a verification branch to check that the result is a valid Philippine zone (50 or 51).
PPCS Zone Assignment Decision Procedure
Flowchart illustrating how to assign a point to its correct PPCS zone by computing the angular distance from the point's longitude to each of the 5 PPCS central meridians and selecting the nearest one. Reinforces the concept that PPCS zone assignment is based on proximity to the central meridian.
PPCS vs UTM System Comparison Flowchart
A side-by-side comparison flowchart showing the key parameters and use cases of PPCS and UTM. Emphasises that datum transformation is required when converting between the two systems. The false easting of 500,000 m is shown as common to both, while differences in zone width, scale factor, and ellipsoid are highlighted.
Timeline of Philippine Geodetic and Coordinate System Development
A chronological timeline showing the evolution of Philippine geodetic coordinate systems and the key laws that govern their use. Helps examinees connect legal knowledge (RA 4374, RA 8560, PD 1529, CA 141) to the historical development of PPCS and PRS92.
Exam strategy
For the PRC Geodetic Engineer board exam on this topic, prioritise these three skills in order: (1) Compute UTM zone number using ⌊(λ+180)/6⌋+1 without error — this formula appears in almost every exam set and must be executed mechanically and correctly, including the floor function step; (2) Assign PPCS zones instantly using the CM table (117, 119, 121, 123, 125 for Zones I–V) by finding the nearest CM to the given longitude; (3) Apply the false easting (500,000 m) correctly to compute grid eastings from CM departures. For the conceptual/legal questions, focus on three contrasts: PPCS k₀=0.99995 vs UTM k₀=0.9996; PRS92/Clarke 1866 vs WGS84; 5 PPCS zones vs 2 UTM zones for the Philippines. In multi-step problems, always write out each computational step — the floor function, the false easting addition, and the final answer with correct units and zone label. Time-saving tip: For any Philippine longitude between 120°E and 126°E, the UTM zone is always 51N; for 114°E to 120°E, it is always 50N — verify with the formula but these benchmarks allow rapid answer checking. Legal questions are typically recall-based: RA 8560 = geodetic engineering law; PD 1529 = land registration (PPCS required); CA 141 = public lands; NAMRIA = official datum authority.