The ENTCHEV GIS Blog: Recent Comments

Comment on Relying on VGI for your mapping needs is like relying on Twitter for your news

SEO — Tue, 16 Mar 2010 12:01:27 GMT

I helped build an Emergency Notification System (ENS) for a 911 district here in Texas.

Comment on Positional Accuracy Part 1

Mike Popoloski — Fri, 12 Mar 2010 21:20:13 GMT

I can't say that a new "GPS" survey is more accurate than a 20yr old survey because so much depends on how the survey was done and for what purpose. Someone can use GPS improperly today and someone 20yrs ago may have done a great job with the older equipment used at that time. Stay tuned for my next post on GPS accuracy issues, there are many.

Also, the word "survey" is often used to cover a lot of things. From your description it sounds like you are talking about a survey showing property boundaries. If that is the case then I can tell you that I rarely use GPS alone to perform certified boundary surveys. Usually the only projects that make sense to use GPS for boundary work are very large tracts in open areas, a few hundred acres and up. It is much more common in my area to use GPS to position a boundary survey in the state plane coordinate system to satisfy a local GIS regulation after the legal survey work is done.

So the smaller boundary survey is still done much like it was 20yrs ago, with an optical survey instrument. Then if the customer wants to pay the extra $$$ we can set a high accuracy GPS baseline on or near the project so the boundary survey can be referenced to the state plane coordinate system. Then we would setup the digital drawing file coordinates and orientation using the GPS coordinate information and label property corners on the map with information to match.

Please understand that boundary surveys are first and foremost a legal investigation that must follow the rules of evidence, including discovery of markers moved by the sweet old lady at the street corner. Legal documents such as deeds or maps on file must be analyzed, etc etc. After that is accomplished then measurements are made to locate the evidence uncovered along with the improvements on the property and anything else that is covered under the contract with the client. GPS is one tool that can be used to make measurements but it has it's strengths and weaknesses like everything else. On smaller projects or ones with tree canopy GPS has limited use in boundary work because the accuracy of GPS is limited in those areas.

Hope this helps.

Comment on Positional Accuracy Part 1

Paul Renaud — Thu, 11 Mar 2010 13:16:42 GMT

Accuracy in surveying with GPS- As a civil servant I am always comparing new surveys againsed 20 year old surveys.Are GPS surveys more accurate? Do new surveys confirm by GPS existing benchmarks and control points or do you start your survey from the street corner benchmark that may have been moved when the sweet old lady planted an ornamental?

Comment on Positional Accuracy Part 2

Russ Kauffman pls GISP — Wed, 10 Mar 2010 17:02:49 GMT

I enjoyed reading the discussion on accuracy, both relative and absolute, as it brought back long ago memories. The discussion and often arguments about which type of accuracy is necessary for effective GIS or perhaps preferably "Geo-Spatial Information Processing" goes back before we even had the word GIS. At the Keystone III AM/FM conference around 1980 I was engaged in a panel dialog with an eminent photogrammetrist about the appropriate type of accuracy needed for a utility AM/FM projects. After the session and much more conversation I concluded that the relative accuracy argument was being pushed by many vendors in the industry for several reasons. most utilities did not have PROFESSIONAL mappers on staff, either surveyor/civil engineers or cartographers; photogrammetric base mapping was expensive and difficult with very large data files, Computers were slow, memory starved and often built fot business/monetary calculations rather than engineering precision. Most software only supported 16 or 32 bit data for a long time, A major vendor only got on board with 64 bit data relatively recently.
I was surprise that the author refers to most surveyors still using relative accuracy, since my experience is that all of the successful surveyors I know are using GPS enabled processes even if the final field work is done with total stations or robotic stations. You can't stay in business with transit and tape approaches any longer. It may very well be that the survey is done in real GPS coordinates , yet published to the client in what appears to be a relative manner. The failure of clients to require metadata in the survey notes hides the methods used.
Another factor that has influenced the discussion is the failure of many end users of GIS data acquisition projects to understand the difference between accuracy and precision.

I am also convinced that most GIS projects are intended to be automated cartography vs. automated spatial models sufficient to engineer solutions from. From my own experience I know that cartographic license is employed to place overhead pole lines at an offset from the streets that places the poles in the front yards when viewed on a high resolution ortho base. This phenomenon occurs because of a software requirement to have a one to one relationship for each phase of the conductor and the graphic occurrence of it.

Thanks for an interesting topic and one that still needs to be discussed and better understood after all these years.

Comment on Positional Accuracy Part 2

atanas entchev — Tue, 09 Mar 2010 14:44:04 GMT

I shared my suspicion with the Wired article's author, Betsy Mason. She promptly responded with a link to the source. 10 feet is correct.

http://www.eurekalert.org/pub_releases/2010-03/osu-rsh030810.php

Comment on Positional Accuracy Part 2

atanas entchev — Mon, 08 Mar 2010 22:51:23 GMT

I also think Wired may have gotten their unit conversions wrong, and the actual shift is 1/10 of a foot.

Comment on Positional Accuracy Part 2

atanas entchev — Mon, 08 Mar 2010 22:44:53 GMT

Whether or not to include categories and/or tags is up to you. They do make the articles easier to find by humans and computers, so I use them now (although I did not in my first year of blogging).

Comment on Positional Accuracy Part 2

Mike Popoloski — Mon, 08 Mar 2010 22:31:59 GMT

Thanks for the complement Atanas! I had not heard of the 10ft shift in Chile. California has to deal with the same issues, though at a much slower rate . Years ago they installed a high accuracy GPS on both sides of the faults out there so they could monitor and update the datum info. Currently all the CORS have published velocity data and I am told that in the future all U.S. geodetic control will have velocity data published with the geodetic control datasheet. Even now when I do a high accuracy control project with CORS I include the epoch date along with the datum info i.e. NAD-83 CORS 2002.

On another note I was wondering if I should be including categories or tags when I publish a blog?

Comment on Positional Accuracy Part 2

atanas entchev — Mon, 08 Mar 2010 20:50:13 GMT

Another great article. Quite coincidentally, Wired publishes a story underscoring (in my opinion) the relevance of relative accuracy:

Chile Earthquake Moved Entire City 10 Feet to the West

http://www.wired.com/wiredscience/2010/03/chile-earthquake-moved-entire-city-10-feet-to-the-west/

Comment on Positional Accuracy Part 2

Morten — Mon, 08 Mar 2010 03:39:07 GMT

I've had customers who would complain that the building outlines we mapped was incorrect because they didn't match their orthophotos, completely ignoring the fact that buildings are often "leaning" to the side, and the outlines matched perfectly at the foot of the buildings. Of course we used it as an opportunity to upsell to "true orthophoto".