Point of Beginning

Letters to the Editor

July 29, 2003
POB readers sound off on topics in recent issues of the magazine.

The Fine Points of Scanning
May 2003

The recent scanning article written by [James Steere, PLS, Michael W. King, PLS, and Ken Moscetti] challenged my understanding of the word “accuracy.” My daily workload revolves around chasing handfuls of microns, and typically when a client has a certain accuracy requirement I’m obligated to use a tool that will provide quality results to meet that requirement.

In the portion of the article dealing with the Collingswood Circle project, it’s mentioned that the accuracy requirements were .01' for hardwork. My query not only centers on why the client even considered this tool since it couldn’t meet their accuracy requirements, but also why the writers saw the project as a success since it was not possible to, nor did it attain the client’s required accuracies. My experience and [that of] my local Cyra vendor [leads us to] concur that 1/4" (6 mm) was the best repeatable accuracy attainable.

Is our profession altering the word “accuracy” to suit the tools they use, or was this a sales gimmick to get the tool into the marketplace, even to the detriment of the resultant design quality of the project?

To clarify my stand, I work at a facility that owns and uses a Cyra scanner.

Bruce Jahn, PLS
via E-mail

Cyra representatives respond:

You are correct in that the official accuracy spec for a Cyrax 2500 is 6 mm, but it is common for users to report results that are more accurate than that spec, as it does in the article mentioned (.01' translates to 0.125" or about 3 mm). Cyra customers typically check their Cyrax results against total station surveys on targets to come up with their own assessments.

Cyra also specifies “modeled” accuracy (e.g. modeling a pipe or a flat surface) of objects to be 2-3 times better than the single point accuracy of 6 mm. This method uses least squares fitting through thousands of sample points to achieve a mathematical representation of the object. The authors from Medina indicated in the article that they would “create cross sections through the meshed point cloud,” which implies an averaged surface. Although this is not considered “modeling,” Medina claims to have had reports from its customers that the result can be “closer to the truth” (more accurate) than their specified single point accuracy of 6 mm.

Editor’s Note
June 2003

The thing that first got me interested in surveying was the historical aspect of it (reading old deeds, looking at old tax maps, finding stones set 150 years ago, etc.). Maybe Sam should be shown evolving from the beginning of surveying in this country to the present. It would show that surveying has always been an important and necessary profession. The “poster surveyors” could start with George Washington (“Wow! The father of our country was a surveyor, too?” says the 8 year old), and end with an American Marine or soldier using GPS equipment during the Iraq war (“Cool!” says the 8 year old). Of course the possibilities in between are endless. You might want to reconsider including a machete with the Sam the Surveyor action figure.

Matt Kirk
via E-mail

Editor’s Note: So noted, Matt. Thanks for the tip.