Setting Datum Elevations

[msquared48]

OK...

Recently got a Garmin for the car and have been using it some. I've noticed though that the elevation listed appears off by 20 to 30 feet or so as I know that I am not underwater when I drive 10 feet above Puget Sound and it registers -20 feet.

Anyone know a way to calibrate the Garmin to a local USGS known datum elevation?

Mike McCann
MMC Engineering
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[johnwm]

You will also see the earth tide effect which changes heights by up to 600mm on a daily basis.

Good Luck
johnwm
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[btrueblood]

IRstuff is mostly correct, the ellipsoid (which is a subset of things called spheroids, so nyah) that models the earth's surface for the GPS system has not changed since 1984.

But before that, it used a spheroid dubbed GRS80. So it has been changed, and may yet change again...but the changes don't affect practical applications for GPS.

WGS-84 has been revised, though, through updates to the geoid, or the geopotential mean sea level model, which is the datum from which altitudes (true altitudes) are measured from. That model is basically a mapping of the geopotential altitudes as deviations from the spheroid/ellipsoid. Again, the lumpiness of earth's gravity does indeed change over time, and these changes affect the altitude of earth topography, as defined by "mean sea level". Whether or not your GPS receiver includes such corrections is debatable, but likely not.

From wikipedia:

"The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).

Many of the original authors of WGS 84 contributed to a new higher fidelity model, called EGM2008.[9] This new model will have a geoid with a resolution approaching 10 km, requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84)."