What does Height Really Mean?

How We Measure Height• Heights are generally measured against one of two

different reference frames (datums).

1. Gravity (geopotential height)

2. Ellipsoid (ellipsoid height)

• These heights are not interchangeable.

• Without site specificgravitation measurementsno transformation betweenthem.

• Orthometric Height

Ellipsoid Height(I know these pictures are spheroids but they look good!)

•Height above a mathematical surface

•Simple, cost effective, time invariant reference surface

Geocentric Datum: Regularly used for height measurements particularly by GPS devices

Local Datum: Not generally used for height measurements

Geoid in Height Measurements•Initially Mean Sea Level used as the reference surface

•Discovery that MSL is not the same height in all places

•Requirement to find a new reference surface…. Geoid

•Geoid is “the equipotential surface of the Earth’s gravity field that nominally defines mean sea level” Meyer et al. 2005.

•Geoid is perpendicular to gravity at all points

•Traditional measurement techniques (e.g. levelling) are heights above this surface

Relating Ellipsoid Height to Orthometric Height

h = H + N

Important to note that h and N are reference ellipsoid specific

In Australia the value of N can be obtained from AUSGeoid98

GPS heights less the value of N provide the orthometric height

This technique is referred to as GPS levelling

Not yet regarded as being as accurate as traditional levelling

Why don’t we just use GPS heights?

• Reference Ellipsoid lacks the subtle variation of the geoid Water flowing uphill

• GPS levelling will improve as geoid models improve (sampling at a finer spatial resolution)

• If everyone used the same reference ellipsoid and it did not change, isn’t that enough?

• 60% of GPS users do not come from a surveying background. (Waite 2005) Why should they doubt height readings from their GPS device.

• Answer to these problems: Real time correction for orthometric height within GPS devices. (Connectivity)

Questions?

http://earth.usc.edu/~geol150/variability/orbitalchanges.html

http://137.122.126.164/website/zoran_reljic/idex/portfolio.html

Meyer, Thomas H.; Roman, Daniel R.; Zilkoski, David B. (2005) What does height really mean? Part II: Physics and Gravity, Surveying and Land Information Science, 65(1), pp. 5-15.

Waite, Lucan (2005) Geoid Modelling, Survey Quarterly, 43 pp. 17-18.

REFERENCES