SME
Surveying
GNSS
Use this for “GNSS gives an ellipsoid height h = 45.320 m where the geoid height N = −28.750 m. What is the orthometric height H?”
Diagram
H = h − N = 45.3 − (−28.8)
H = 74.1 m
Here the geoid lies below the ellipsoid (N < 0), so H is more than h.
This is an example. Type your own values to start a new problem.
H (m)
=
h (m)
−
(
N (m)
)
Formulas
H
=
h
−
N
74.07 = 45.32 − (−28.75)
Assumptions
•
h is what GNSS measures: the height above the ellipsoid, a smooth model of Earth’s shape.
•
N comes from a geoid model; it is negative across most of the US, where the geoid lies below the ellipsoid.
•
H is the orthometric height above the geoid (mean sea level), the elevation surveyors use.
Step-by-step
WE KNOW
h = 45.32 m, N = −28.75 m
FIND
orthometric height (H)
Step 1 · Find orthometric height (H)
Use H = h − N
Measure
from
the
geoid
instead
of
the
ellipsoid:
take
the
geoid’s
height
off
h
.
H
=
45.32
−
(−28.75)
H
=
74.07
m
Check
Put the numbers back in. Both sides should match.
74.07 = 45.32 − (−28.75) ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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