Comments on: Ellipsoid/Spheroid – Our Oblate Spheroid Planet Earth

By: Mark Seidenberg

Mark Seidenberg — Fri, 11 Jun 2021 03:09:31 +0000

The Alaska DNR requested I convert NAD27 to NAD83 (2011) to do land recombination to subdivided land on Wrangell Island (know between 12 August 1881 to 29 July 1901 as New Columbia Land. Then I was advised by Lt. Owen and Dr. Dennis at NOAA that both Wrangell Island and Saint Paul island in Alaska never had extended NAD27 to them, so 178^31’W longitude was not fixed by NAD27. An inquiry with the NGA came up with the fact that 1505 NR1, 2-1 in 1966 used Hayford of 1910. Can one convert the 178^31’W longitude from Hayford Ellipsoid to NAD83 (2011)?

By: Richard Rhodomoyer

Richard Rhodomoyer — Sun, 16 May 2021 00:11:39 +0000

I’ve done dozens of land surveys with my Dad who is a surveyor and engineer.
We’ve done rails to trails topo surveys over 75 miles. Never adjusted to any curvature because there isn’t any! And no, our instruments didn’t adjust either. How can you adjust for curvature when nobody even agrees with the actual shape of the earth?
That includes the professionals messaging back and forth on this website.
There are no actual photos of earth from space. Only CGI. NASA admits this claim.

By: Donald Mulcare

Donald Mulcare — Wed, 25 Nov 2020 09:24:49 +0000

For Ayebare Mary, this links to information about the Earth Gravitation Model of 2008.

https://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_wgs84.html

As for the field of study most associated with geoid modeling and gravity, it is geodesy, specifically physical geodesy. Geophysicists also contribute.

A good, accessible text on geodesy is: Geodesy for Geomatics and GIS Professionals by Elithorp and Findorf (citation from memory).

There are more rigorous texts as,well,as lecture notes and reports at The Ohio State University and University of New Brunswick to name two.

By: GISGeography

GISGeography — Tue, 08 Sep 2020 17:10:17 +0000

In reply to Donald Mulcare. Thanks, it's updated

By: Donald Mulcare

Donald Mulcare — Tue, 08 Sep 2020 16:31:47 +0000

The value for the inverse of flattening you report for the WGS84 datum is wrong.

The inverse of flattening is:298.257223563 NOT the value you show.

The semi-minor axis (b) you show for WGS84 truncates the official value in NIMA TR 8350.2 third edition. The value is 6356752.3142 meters.

As you know, truncating values impacts results using them. The significant figures used in the Technical Report are intended to insure agreement in numerical results. As the semi-minor axis is NOT a defining parameter, it is computed using a and f. Given a and b one can also compute the flattening or its inverse.

By: Donald Mulcare

Donald Mulcare — Tue, 08 Sep 2020 05:46:09 +0000

To Derrick Chella

The following site run by the US National Geodetic Survey can perform these conversions for either single of multiple points in the US:. https://geodesy.noaa.gov/NCAT/

By: Donald Mulcare

Donald Mulcare — Tue, 08 Sep 2020 05:39:09 +0000

In the section “What is an ellipsoid in GIS” you show more of a circle than an ellipsoid. You also show the semi-major axis as larger than the semi-minor axis. Your illustration does not bring the semi-major axis back to the spin axis.

Your article also shows incorrect WGS 84 parameters for both b and 1/f. They should be:

Semi-minor axis (b) 6356752.3142
Inverse of flattening should be: 298.257223563

Strictly speaking both the WGS 84 and GRS80 reference ellipsoids are defined by

Semi-major axis (a)
Earth gravitational constant (GM)
Angular velocity of the Earth (mu)
Reciprocal of flattening (1/f)

Retired geodesist

By: Ayebare Mary

Ayebare Mary — Mon, 14 Oct 2019 06:31:54 +0000

Okay thanks but I need to know the earth gravitational models

By: Mary

Mary — Thu, 26 Sep 2019 01:17:45 +0000

I think I understand the concept of a Geoid representation, but I don’t understand why the Geoid image looks lumpy – when you look at an image of Earth from space, it looks “smooth” and basically spherical (Even if we know it isn’t actually a perfect sphere)

By: GISGeography

GISGeography — Sun, 21 Jul 2019 02:22:07 +0000

In reply to Greg Esres.

I’d say it falls under geomatics. Here’s a brief explanation on the topic – https://gisgeography.com/geodesy/