Letters to the Editor

GPS Realization

GPS is realized in X, Y, Z (Cartesian format), not lat, lon, height, right? Lat, lon, height is also a conversion by the software we use.

Thank you for your response.

Edward Glawe
Montgomery County, MD

Mr. Glawe,
You are correct. The actual coordinate system realized in the use of GPS is an X, Y, Z Cartesian coordinate system called ECEF (Earth-centered, Earth-fixed) where the origin is defined as the center mass of the Earth. Our software then converts the three-dimensional coordinates to latitude, longitude, and ellipsoid heights using the GRS80 ellipsoid model before then converting to other useful coordinate systems. State Plane coordinates are derived from latitude and longitude. I hope that I did not confuse anyone. I should have been clearer on that end.

When using GPS for boundary surveys, the scale factor is also really important and often overlooked. Most meets and bounds describing real property within a deed of conveyance are based on a flat plane coordinate system developed by the original surveyor in his calculations of the field run survey. When I retrace an original survey, I must follow in the original surveyor’s steps; therefore, if I use GPS as a form of measurement for my field work, I must make sure that the coordinate system I use to measure the relative distance between monuments is the same as used by the original surveyor.

Sincerely,
Paul G. Weatherford, RLS

 
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Photo Safety?

Mr. Sandler,

The photo, right, was taken from your June 2011 edition. Aside from the fact the worker appears to be wearing tennis shoes (which clashes mightily with his hard hat and safety glasses), the trench he’s standing in is subject to collapse (as it did on the opposite side) and might engulf or kill the worker.

I assume his employer might argue the soil composition is such that shoring isn’t necessary. However, if the Occupational Safety and Health Administration (OSHA) ever got hold of this photo, they would make him prove it. Otherwise, it’s a great article.

Dave Rentfrow, CST Level IV
Orlando, MD

Mr. Rentfrow,

Thanks for reading our magazine so carefully and for writing in. We did investigate your concerns. According to the surveyor in the photograph, that photo was a posed shot for the article. Nevertheless, he reports that he was wearing steel-toed hiking shoes and that the trench was open in the foreground. All is well.

Shelly Cox

» Back to our August 2011 Issue

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