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It is true that the parameters of any state's particular SPC system are usually installed in data collectors by the manufacturers to allow a coordinate transformation from GPS coordinates to SPC. But the transformation is only as accurate as the controlling points used, which you iterated in your article. Many local governments and of course state agencies beginning in the early 1990s undertook base mapping operations in support of GIS. I have been involved in that process for two counties now that utilized static GPS and high-precision surveys to create the base maps. There was an enormous amount of very good work performed.
The area that gets a bit lost in translation, or should I say transformation, is that these are horizontal control networks with a significant amount of adjustment. The accuracy of these systems within themselves is for the most part very good, and they are on SPC. But to match these networks, you cannot rely on the built in transformations supplied by the equipment manufacturers alone, as they do not take into account the local adjustments.
An example I use often is with the orthophotos available on many local GIS. Along with all the standard inherent distortions that come with these, is the fact that most are now controlled by airborne GPS, having a very limited relationship to the initial controlling points of the base maps for the local systems. Another issue is the GPS epoch utilized. As we know, the values gained by GPS now compared to few years ago do not match due in part to plate tectonics. Manufacturers, in their efforts to make coordinate transformations "quicker" to perform, now offer several options in their software packages, from single point to classic seven-parameter methods. However, I have found most quite lacking in their documentation on the various methods.
I was fortunate to earn my BS in Surveying Engineering (now Geomatics Engineering) from CSU Fresno. The degree was split between photogrammetry and surveying. Although most of my career has been in the surveying end, the photogrammetry education has proven invaluable. As I like to say, in that part my education, error analysis, sources of error, and coordinate transformation was beaten into us! But that has served us well coming into employment in the early 1990s at the same time GIS was demanding base mapping and GPS was taking off.
The point I always try to get across is that SPC is merely a coordinate system. And coordinates alone, whether SPC or 10,000, 10,000, do not make something accurately located. Just because your data collector can instantly display an SPC, in reality has no relationship to how accurate it may be. Your article seemed to be pointed to the responsibility of surveyors to use correct procedures for obtaining and using SPCs. I would add that professional surveyors should always be at the forefront of this issue, both knowing all factors that go into obtaining accurate SPCs for the area of their work, and be the experts in debunking the myth that exists outside surveying that [using] state plane coordinates means it is accurate.
-- Ken Paul, PLS, Vancouver, WA
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