Memory AnchorsGELE · GeodesyReal content

GELE Geodesy — The Geoid, Gravity and HeightsMemory Anchors

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Memory anchors and mnemonic tricks for The Geoid, Gravity and Heights. If you find yourself forgetting key facts from this chapter during GELE mocks, these anchors are your fix. Built for Professional Regulation Commission (PRC) — Board of Geodetic Engineering's question style and the time pressure of the GELE 2026.

Exam context

Professional Regulation Commission (PRC) — Board of Geodetic Engineering runs the Geodetic Engineer Licensure Examination on September 2026. Its Geodesy section sits under a "Core" weighting, and The Geoid, Gravity and Heights is the 5th chapter in the 6-chapter GELE Geodesy rotation. The GELE passing mark is 70% weighted average, no sub-test below 50%, and the most recent 2026 paper drew about a meaningful share of questions from Geodesy.

The Geoid, Gravity and Heights - Memory Anchors

Memory anchors dramatically improve recall by linking abstract geodetic concepts to vivid, emotionally engaging mental images, stories, and patterns. Research shows that information encoded with multiple sensory and emotional hooks is retained up to 6× longer than rote memorization. For the PRC Geodetic Engineer board exam, where formulas like H = h − N can mean the difference between passing and repeating, these anchors give you instant retrieval under pressure. Use mnemonics for formulas, analogies for concepts, and micro-stories for processes — then test yourself with the revision game. The goal: when you see 'GNSS height' on the board exam, your brain fires immediately: 'That's h, not H — I need the geoid model!'

Anchors

Topic

Three Surfaces of Geodesy

Concept

The three surfaces in geodesy: Ellipsoid, Geoid, and Terrain

Anchor Id

Difficulty

easy

Memory Aid

Remember 'EGT' — 'Engineers Go Topside!' The Ellipsoid is the math model underground (smooth), the Geoid is gravity's sea-level surface (wavy), and the Terrain is where engineers actually walk on top. Think of a Filipino construction engineer saying 'Engineers Go Topside!' as they climb a surveying hill — first they pass the imaginary math surface (E), then the wavy sea-level surface (G), then finally reach the actual ground (T).

Anchor Type

acronym

Why It Works

The acronym EGT forms a memorable phrase tied to a visual action (climbing), and the three layers reinforce the hierarchy from mathematical to physical to real.

Example Usage

Board exam asks: 'What are the three reference surfaces in geodesy?' — Recall EGT: Ellipsoid (math), Geoid (gravity/MSL), Terrain (physical ground).

Recall Trigger

Think: 'Engineers Go Topside!'

Topic

Height Equation h = H + N

Concept

The fundamental height equation: h = H + N

Anchor Id

Difficulty

easy

Memory Aid

Chant this: 'Little h is what GNSS sees, Big H is what engineers need, N is the gap in between — add them together and you're clean!' Or use the Filipino flavor: 'h equals H plus N — huwag kalimutan, 'yan ang laman!' (Don't forget, that's the content!). The lowercase h is the 'humble' GNSS output, capital H is the 'Hero' elevation engineers use, and N is the 'Nudge' the geoid gives.

Anchor Type

rhyme

Why It Works

Rhyme and rhythm encode the formula in procedural memory (like song lyrics), making it instantly retrievable under exam stress. The H=Hero, h=humble, N=Nudge keyword associations link each variable to a distinct image.

Example Usage

When given h = 52.30 m and N = −30.10 m, recall the chant → h = H + N → rearrange to H = h − N = 52.30 − (−30.10) = 82.40 m.

Recall Trigger

Sing the chant: 'Little h, Big H, N in between'

Topic

Orthometric Height

Concept

Orthometric height H is what engineering uses (height above the geoid/MSL)

Anchor Id

Difficulty

easy

Memory Aid

Orthometric height H is like your PRC license rating — it's the number that actually MATTERS in the real world. The ellipsoidal height h is like your raw test score before scaling — it's technically correct but not immediately useful to your employer. When a client asks 'How high is this building?' they want H, not h. The geoid undulation N is the 'conversion factor' between your raw score and your official rating. Just as you always convert scores before reporting, always convert h to H before reporting elevations.

Anchor Type

analogy

Why It Works

Connecting H to a familiar high-stakes scenario (PRC licensing) creates emotional relevance for Filipino examinees, making the distinction between h and H personally meaningful and memorable.

Example Usage

Exam question: 'A GNSS survey yields h = 40 m at a tidal flat. What elevation do you report?' Recall analogy → You need H, not h. Apply H = h − N with a geoid model.

Recall Trigger

Think: 'PRC official rating = H; raw GNSS score = h'

Topic

Geoid Undulation N

Concept

Geoid undulation N — the geoid can be above OR below the ellipsoid

Anchor Id

Difficulty

medium

Memory Aid

Picture the Philippine Sea: the ellipsoid is a perfectly smooth billiard ball. The geoid is a water-filled balloon stretched over it — sometimes bulging out toward you (positive N, like over dense ocean ridges), sometimes sucked in (negative N, like over lighter crust under the Philippines). In the Philippines, N is approximately −3 m to −30 m in many areas — meaning the geoid SINKS below the ellipsoid. Visualize the balloon deflating slightly every time you cross to Mindanao — that's negative N territory. So H = h − (negative N) → H becomes LARGER than h.

Anchor Type

visual_association

Why It Works

The balloon-over-billiard-ball image is concrete, spatially vivid, and physically accurate. Attaching it to Filipino geography makes it personally relevant and spatially anchored.

Example Usage

Given h = 52.30 m, N = −30.10 m → H = 52.30 − (−30.10) = 82.40 m. The 'deflated balloon' reminds you N is negative here, so H > h.

Recall Trigger

Visualize: deflating balloon on a billiard ball over the Philippine map

Topic

Geoid Definition

Concept

The geoid is an equipotential surface of Earth's gravity field

Anchor Id

Difficulty

medium

Memory Aid

The geoid is like the still surface of Laguna de Bay on a perfectly calm morning with no wind. Water always finds its own level — it flows downhill until it reaches zero slope, which means zero net gravitational force sideways. That calm flat water surface IS an equipotential surface: gravity pulls equally at every point on it. The geoid is just that imaginary 'calm water surface' extended under all the land of the Philippines and the entire Earth. If you poked a canal through Luzon from Manila Bay to Lingayen Gulf, water would flow until it matched the geoid — not the ellipsoid.

Anchor Type

analogy

Why It Works

Laguna de Bay is a familiar Filipino landmark. Still water as an equipotential is a perfect physical analogy that makes the abstract concept concrete and self-explaining.

Example Usage

Board exam asks: 'What physical property defines the geoid?' Recall Laguna de Bay image → The geoid is the equipotential surface of Earth's gravity field that best fits global mean sea level.

Recall Trigger

Picture: Still water at Laguna de Bay at dawn

Topic

GNSS Height vs Orthometric Height

Concept

GNSS gives ellipsoidal height h, NOT orthometric height H

Anchor Id

Difficulty

easy

Memory Aid

Story: 'Engr. Marites just got her shiny new GNSS receiver and proudly announces: My benchmark elevation is 40 meters! Her supervisor Engr. Berto frowns: 40 meters above WHAT, Marites? The ellipsoid? That's in space! You need a geoid model — go get EGM2008 or PGM2019 and subtract N!' Marites blushes: 'Ay, nako! I confused h with H again!' — Berto sighs: 'GNSS gives h. Engineering needs H. Never forget.' Moral: The GPS/GNSS antenna doesn't know where the ocean is — it only knows the ellipsoid.

Anchor Type

micro_story

Why It Works

A relatable Filipino workplace story with named characters and dialogue creates an episodic memory that is much easier to retrieve than a bare fact. The emotional hook (embarrassment + correction) reinforces the lesson.

Example Usage

Exam asks: 'What type of height does a GNSS receiver directly output?' Recall the Marites story → GNSS outputs ellipsoidal height h, not orthometric H.

Recall Trigger

Remember Marites proudly announcing the wrong height to Berto

Topic

Geoid Undulation Range

Concept

Geoid undulations range ±100 m relative to the ellipsoid globally

Anchor Id

Difficulty

easy

Memory Aid

Remember: '±100 m — One Football Field.' The tallest structure on a standard Philippine football (soccer) field is about 100 m (if you stacked the stands). The geoid bobs up and down by at most one football field's height anywhere on Earth relative to the ellipsoid. This is huge in engineering terms but tiny compared to Earth's radius (~6,371 km). Chunk it: GEOID UNDULATION → ±100 m → ONE FOOTBALL FIELD → globally.

Anchor Type

chunking

Why It Works

Chunking large numbers into everyday spatial references (football field) converts abstract magnitude into an immediately visualizable scale. The chain of associations is short and retrieval is fast.

Example Usage

Board exam asks: 'What is the approximate range of geoid undulation globally?' → Recall football field → ±100 m relative to the ellipsoid.

Recall Trigger

Think: '±100 m = one football field tall'

Topic

Gravity Variation

Concept

Gravity varies from ~9.78 m/s² at the equator to ~9.83 m/s² at the poles

Anchor Id

Difficulty

easy

Memory Aid

Use '9.78 E, 9.83 P' — remember: 'Eight at the Equator, Three at the Poles' (the last two digits: 78 has 8, 83 has 3). Or say: 'The equator is lazy (9.78) — you weigh LESS there — the poles are energetic (9.83) — you weigh MORE there.' As a Filipino, you're near the equator at ~9.78 m/s². Travel to the North Pole and you'd weigh about 0.5% more — enough to notice on a sensitive scale. Remember: E = 78, P = 83.

Anchor Type

mnemonic

Why It Works

The equator-lazy/poles-energetic contrast creates a memorable narrative. The numerical pattern (78→83 as you go from E to P) encodes both values in sequence.

Example Usage

Exam: 'What is approximate gravity at the equator vs poles?' → Recall E-lazy-78 / P-energetic-83 → 9.78 m/s² equator, 9.83 m/s² poles.

Recall Trigger

Think: 'Equator lazy 78, Poles energetic 83'

Topic

Sign of Geoid Undulation

Concept

The sign rule: When N is negative, H > h (orthometric height exceeds GNSS height)

Anchor Id

Difficulty

medium

Memory Aid

Picture a seesaw in a Philippine elementary school. Label the left seat 'h' (GNSS) and the right seat 'H' (orthometric). N is the fulcrum shift. When N is NEGATIVE (fulcrum shifts left, geoid below ellipsoid), the right side 'H' goes UP — it becomes bigger than h. Mathematically: H = h − N. If N = −30, then H = h − (−30) = h + 30 > h. The seesaw tilts: h goes down (it was given the negative N weight), H goes up. Draw this seesaw in your mind every time you see a negative N.

Anchor Type

visual_association

Why It Works

The seesaw is a universal childhood image that physically demonstrates the mathematical effect of subtracting a negative number, making the abstract sign rule concrete and physically intuitive.

Example Usage

Given h = 52.30 m, N = −30.10 m → Recall seesaw: N is negative so H > h → H = 52.30 − (−30.10) = 82.40 m. Confirmed: H > h. ✓

Recall Trigger

Visualize: seesaw with h and H, N as the fulcrum shift

Topic

Computing Geoid Undulation

Concept

Deriving geoid undulation: N = h − H (from GNSS + leveling benchmark)

Anchor Id

A10

Difficulty

medium

Memory Aid

N is what's LEFT OVER after you subtract the levelled height from the GNSS height. Think of it like comparing two measuring tapes at a benchmark: Tape 1 (GNSS) measures from the ellipsoid up — it reads h. Tape 2 (Spirit Level) measures from the geoid up — it reads H. The difference between the two tapes at the SAME point is exactly N = h − H. The geoid undulation is literally the offset between the two tape zero-points at that location. Use any established benchmark in the Philippines (NAMRIA BM) that has both H and h to compute local N.

Anchor Type

analogy

Why It Works

The two measuring tapes analogy is physically exact and connects to familiar survey equipment, making the derivation self-evident rather than memorized.

Example Usage

Given H = 112.20 m (leveled), h = 124.50 m (GNSS) → Recall two-tape analogy → N = h − H = 124.50 − 112.20 = 12.30 m.

Recall Trigger

Picture: two measuring tapes at a NAMRIA benchmark — the difference in their zeros is N

Topic

Geopotential Numbers

Concept

Geopotential numbers — rigorous height definition combining leveling + gravity

Anchor Id

A11

Difficulty

hard

Memory Aid

A geopotential number is like your total electric bill — it's not just the 'height' of water in the tank (leveling) but the actual ENERGY (gravity × distance) needed to pump it there. Two tanks at the same levelled height but different latitudes actually have different water pressures because gravity differs. The geopotential number C = g × H captures both — it's the 'true energy height.' For most PRC board exam purposes, just remember: geopotential = leveling + gravity, and it's the rigorous basis for orthometric and dynamic heights.

Anchor Type

analogy

Why It Works

The electric bill / water pressure analogy connects an abstract energy concept to a familiar utility experience. The formula C = gH provides the mathematical hook.

Example Usage

Board exam asks: 'What accounts for the difference between orthometric and dynamic heights?' Recall electric bill analogy → Geopotential numbers incorporate gravity variation; dynamic heights use a standard gravity value.

Recall Trigger

Think: 'Electric bill = usage × rate. Geopotential = g × H'

Topic

Cause of Geoid Undulation

Concept

The geoid undulates because of mass-density variations in the Earth's interior

Anchor Id

A12

Difficulty

medium

Memory Aid

Story: 'Imagine gravity as a very bossy crowd control officer at Divisoria market. Where more people (mass) are crammed — like dense iron-ore deposits under the crust — the officer pulls water (and the geoid surface) DOWN toward the crowd, creating a geoid HIGH. Where fewer people gather — like a lighter ocean crust — the officer is relaxed, and water naturally rises less, creating a geoid LOW. The geoid surface is just the water following the gravity officer's commands across the entire Earth.' Mass concentrations pull the geoid toward them; mass deficits push it away.

Anchor Type

micro_story

Why It Works

Divisoria market is iconic in Filipino urban culture. The 'crowd control officer = gravity' metaphor is vivid, locally relevant, and physically accurate — denser mass attracts the geoid toward it.

Example Usage

Board exam: 'Why does the geoid undulate relative to the ellipsoid?' → Recall the Divisoria crowd → Because mass-density variations in the Earth's interior cause spatial variations in the gravity field, pulling the equipotential geoid surface up (geoid highs) or down (geoid lows).

Recall Trigger

Picture: gravity as the Divisoria crowd control officer directing the geoid surface

Topic

Gravity Corrections — Free-Air

Concept

Free-air correction — gravity decreases with elevation (away from Earth's center)

Anchor Id

A13

Difficulty

medium

Memory Aid

Free-air correction is like phone signal strength — the farther you are from the cell tower (Earth's center), the weaker the signal (gravity). You don't need to know what's between you and the tower; just your distance matters. Every time you climb 1 m higher above the geoid, gravity drops by about 0.3086 mGal. Remember '0.3086 mGal/m' using: '308.6 — Three-Oh-Eight-Point-Six — phone signal drops 308 units per floor as you go up a skyscraper.' Free-air = 'free' of mass effects — just altitude.

Anchor Type

analogy

Why It Works

Signal strength analogy is universally familiar to Filipino students. The 'free = no mass' keyword distinction is memorable and conceptually accurate.

Example Usage

Exam: 'The free-air gravity gradient is approximately ___.' → Recall phone signal dropping → −0.3086 mGal/m above the geoid.

Recall Trigger

Think: 'Phone signal drops as you climb — that's free-air correction'

Topic

Gravity Corrections — Bouguer

Concept

Bouguer correction — accounts for the gravitational attraction of the rock mass between the station and the geoid

Anchor Id

A14

Difficulty

medium

Memory Aid

Remember: 'BOUGUER = Big Ugly Rock Under Gravity Estimates Require' — that is, the Bouguer correction deals with the Big Ugly Rock (the terrain slab) sitting under your gravity station. The standard Bouguer plate correction is −0.1119 mGal/m × rock density (in g/cm³). For standard crustal rock (density 2.67 g/cm³): −0.1119 × 2.67 ≈ −0.1967 mGal/m. After Bouguer correction, gravity anomalies reveal deeper mass variations. Key word: BOUGUER = ROCK slab correction.

Anchor Type

mnemonic

Why It Works

The BOUGUER acronym is intentionally silly (Big Ugly Rock) which triggers amusement — emotion enhances encoding. Linking the name to 'rock' makes the physical meaning self-evident.

Example Usage

Exam: 'Which gravity correction accounts for the slab of rock between the station and the datum?' → Recall Big Ugly Rock → Bouguer correction.

Recall Trigger

Think: 'Big Ugly Rock = Bouguer correction'

Topic

Need for Geoid Model in GNSS

Concept

You CANNOT convert GNSS ellipsoidal heights to orthometric heights without a geoid model

Anchor Id

A15

Difficulty

easy

Memory Aid

Story: 'The DPWH engineer submitted a flood-risk map using raw GNSS heights h without applying a geoid model. The low-lying areas near Manila Bay appeared 30 m higher than they actually were (because N ≈ −30 m in Manila). The map declared Manila Bay coast as SAFE from flooding. NAMRIA reviewed it, applied EGM2008 geoid undulations, and found the actual orthometric heights were 30 m LOWER — the entire coastal zone needed urgent flood protection. The engineer's GNSS-only map almost caused a catastrophic policy error. Lesson: No geoid model → no valid elevation → no engineering decision.'

Anchor Type

micro_story

Why It Works

A disaster-scenario story with real Philippine context (Manila Bay, NAMRIA, DPWH) creates an emotionally vivid memory. The consequences make the rule unforgettable.

Example Usage

Exam: 'Can GNSS directly yield orthometric heights?' → Recall Manila Bay flood story → No. GNSS gives ellipsoidal h. A geoid model (e.g., EGM2008, PGM2019) is required to compute H = h − N.

Recall Trigger

Remember: Manila Bay flood map disaster from skipping the geoid model

Topic

Spirit Leveling and H

Concept

Spirit leveling gives orthometric heights H directly (referenced to MSL)

Anchor Id

A16

Difficulty

easy

Memory Aid

Spirit leveling is like filling a long, flat trough with water from the sea and measuring how high the water surface sits at each endpoint. The water (gravity) defines the 'zero' — Mean Sea Level at the tide gauge. Every leveled benchmark in NAMRIA's vertical control network gives you H directly: the height above that MSL zero. GNSS skips the water — it measures from the mathematical ellipsoid in space, ignoring where the ocean surface actually is. Leveling uses water; GNSS uses math. For elevations, trust the water.

Anchor Type

analogy

Why It Works

The water trough analogy is physically exact — spirit leveling IS essentially following the gravity equipotential surfaces step by step. Contrasting it with GNSS's 'math in space' sharpens the distinction.

Example Usage

Exam: 'Which surveying method directly produces elevations referenced to MSL?' → Recall water trough → Spirit (differential/geodetic) leveling.

Recall Trigger

Visualize: a long trough of water from the ocean to the benchmark = spirit leveling

Topic

Philippine Vertical Datum

Concept

In the Philippines, the vertical datum is Mean Lower Low Water (MLLW) or Mean Sea Level at key tide gauges per NAMRIA

Anchor Id

A17

Difficulty

medium

Memory Aid

Remember: 'NAMRIA Sets the Vertical Zero.' For hydrographic charts, the Philippines uses MLLW (Mean Lower Low Water) — the lowest tidal level for navigation safety. For topographic/geodetic work, NAMRIA uses mean sea level at established tide gauges. Mnemonic for MLLW: 'My Little Level Water' — the lowest safe tide level for ships. When the board exam asks about vertical datum in the Philippines, always mention NAMRIA and distinguish hydrographic (MLLW) from topographic (MSL) applications.

Anchor Type

mnemonic

Why It Works

The NAMRIA anchor and 'My Little Level Water' acronym create two retrieval paths to the same fact. The hydrographic vs topographic distinction prevents exam confusion.

Example Usage

Exam: 'What is the vertical datum for hydrographic charts in the Philippines?' → Recall MLLW → Mean Lower Low Water, as adopted by NAMRIA for Philippine navigational charts.

Recall Trigger

Think: 'NAMRIA = vertical zero keeper; MLLW = My Little Level Water for ships'

Topic

Dynamic Heights

Concept

Dynamic heights — same dynamic height means water flows between the two points

Anchor Id

A18

Difficulty

hard

Memory Aid

Dynamic heights are like the floor numbers in a perfectly level building. Floor 5 in Wing A and Floor 5 in Wing B are at exactly the same gravitational potential — no water flows between them. But two benchmarks with the same ORTHOMETRIC height at different latitudes are NOT at the same potential (because g differs) — water WOULD flow between them. Dynamic heights use a standard gravity value so that equal dynamic height always means 'no flow.' Think: Dynamic = Hydrodynamic — flow stops only when dynamic heights are equal.

Anchor Type

analogy

Why It Works

The building floor analogy creates an intuitive sense of 'same level = no flow,' which is the physical definition of equal potential. The hydrodynamic connection makes dynamic height self-defining.

Example Usage

Exam: 'What height system ensures that equal heights correspond to no water flow?' → Recall building floors → Dynamic heights (using standard gravity and geopotential numbers).

Recall Trigger

Think: 'Dynamic = Hydrodynamic = no-flow guarantee'

Topic

Ellipsoid vs Geoid

Concept

The ellipsoid (WGS84/GRS80) is a smooth mathematical surface; it does NOT follow gravity

Anchor Id

A19

Difficulty

medium

Memory Aid

Picture a perfectly smooth marble egg (the ellipsoid) sitting inside a lumpy, slightly deflated volleyball (the geoid), which is itself covered by crinkled aluminum foil (the terrain). The marble egg is perfect mathematics — no bumps, no gravity variations, just semi-major axis a and flattening f. The volleyball follows gravity and water. The foil is the real ground. WGS84: a = 6,378,137 m, f = 1/298.257. Whenever you see WGS84 on a GNSS output, picture the smooth marble egg — it's giving you h above that egg, not above the ocean.

Anchor Type

visual_association

Why It Works

The three nested objects (marble egg, volleyball, foil) create a powerful spatial memory for the three geodetic surfaces and their relative roughness. WGS84 parameters attached to the egg image help remember the numbers.

Example Usage

Exam: 'WGS84 is an example of what geodetic reference surface?' → Recall marble egg → A reference ellipsoid — a smooth mathematical surface of revolution, NOT an equipotential surface.

Recall Trigger

Visualize: smooth marble egg (WGS84 ellipsoid) inside lumpy volleyball (geoid)

Topic

Sign Convention — Geoid Undulation

Concept

The common board-exam pitfall: double negative in H = h − N when N is negative

Anchor Id

A20

Difficulty

medium

Memory Aid

Use the 'TWO NEGATIVES CANCEL' battle cry: When N is negative, H = h − (−|N|) = h + |N|. So H is BIGGER than h. Remember the Filipino expression: 'Hindi hindi = oo!' (Not not = yes!) — two negatives make a positive. Every time N has a minus sign, shout mentally: 'Hindi hindi = oo! H is bigger than h!' This prevents the most common PRC board-exam arithmetic error in heights questions.

Anchor Type

mnemonic

Why It Works

The Filipino language negation rule ('hindi hindi = oo') creates a culturally resonant mnemonic that directly applies to the mathematical sign rule, making it both linguistically and mathematically encoded.

Example Usage

Given h = 52.30 m, N = −30.10 m → Recall 'hindi hindi = oo' → H = 52.30 − (−30.10) = 52.30 + 30.10 = 82.40 m. H > h. ✓

Recall Trigger

Think: 'Hindi hindi = oo! Two negatives = H gets bigger'

Revision Game

Ellipsoidal height (h)

Clue

I am what GNSS gives you directly, but engineers cannot use me alone for construction elevations. What am I?

Memory Link

Recall Marites story (A6) — Marites proudly announced the wrong height because she used raw GNSS output without converting to H.

The Geoid

Clue

I am the wavy gravity surface that hugs global mean sea level. I undulate ±100 m globally. I am the zero for all engineering elevations. What am I?

Memory Link

Recall the Laguna de Bay still-water analogy (A5) and the ±100 m = one football field chunk (A7).

H = 82.40 m. H > h because N is negative — subtracting a negative adds the magnitude. Hindi hindi = oo!

Clue

When N = −30 m and h = 52.30 m, what is H? And why is H BIGGER than h here?

Memory Link

Recall the seesaw analogy (A9) and the 'hindi hindi = oo' mnemonic (A20).

Free-air correction

Clue

I am the height correction that accounts for only the elevation of a gravity station above the geoid, treating the space between as empty. My gradient is 0.3086 mGal/m. What am I?

Memory Link

Recall the phone signal analogy (A13) — signal drops as you go up, just like gravity. Free = free of rock mass.

NAMRIA (National Mapping and Resource Information Authority)

Clue

I am the Philippine government agency responsible for establishing and maintaining the national geodetic vertical control network and geoid model. Who am I?

Memory Link

Recall the Manila Bay flood story (A15) — NAMRIA reviewed and corrected the GNSS-only elevation map using EGM2008.

N = h − H = 124.50 − 112.20 = 12.30 m. Positive N means the geoid is ABOVE the ellipsoid here.

Clue

A benchmark has H = 112.20 m from leveling and h = 124.50 m from GNSS. What is the geoid undulation N? Is the geoid above or below the ellipsoid here?

Memory Link

Recall the two measuring tapes at a NAMRIA benchmark analogy (A10) — the gap between the two tape zeros = N.

Dynamic height (based on geopotential numbers)

Clue

I give rigorous heights by combining spirit-leveling increments with gravity observations. Water flow between two points stops only when I am equal at both ends. What type of height am I?

Memory Link

Recall the building floor analogy (A18) — same floor number = no flow. And the electric bill analogy (A11) — geopotential = g × H.

FALSE. In the Philippines, N is typically negative (approximately −3 to −30 m), so h ≠ H even at sea level. At true MSL, H ≈ 0 but h = N (which is negative), so h < 0 — the ellipsoid is ABOVE the geoid there.

Clue

True or False: On WGS84, the ellipsoidal height h and orthometric height H are always equal at sea-level locations in the Philippines.

Memory Link

Recall the deflating balloon visual (A4) and the marble egg in the volleyball analogy (A19).

γ = normal gravity on the reference ellipsoid (m/s²); φ = geodetic latitude; 9.7803267715 = normal gravity at equator on GRS80; 0.0052790414 = gravity flattening coefficient. This is the theoretical gravity value for a point on the ellipsoid surface, used as the reference to compute gravity anomalies from observed gravity.

1. Equipotential surface of Earth's gravity field
2. Best fits global mean sea level
3. Undulates ±100 m relative to the ellipsoid
4. Orthometric heights are measured from it
5. Defined by mass-density variations inside the Earth

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GELE Geodesy — The Geoid, Gravity and HeightsMemory Anchors

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The Geoid, Gravity and Heights - Memory Anchors

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