Are You Working From the Wrong End of Data?
Trying to obtain survey-grade accuracy from GIS data is working from the wrong end of data. I equate the mindset as trying to get 1-second accuracy out of a 3-second instrument. Yes, the surveyor could turn multiple sets of angles and average the values to get a more confident result, but it’s still a 3-second measurement providing the values. But now aren’t we talking about precision? Repeatability? Survey-grade data is capable of centimeter-level accuracy (sub-tenth) and GIS-grade data is sub meter. Can you smarten up GIS grade to survey grade? If the accuracy of the information is not there, it’s just not there. But you can certainly dummy down survey grade to the less-accurate needs of GIS.
When it comes to GIS grade vs. survey grade, why not collect the highest quality, most accurate, most complete information? Many would say collecting survey-grade data for a GIS project is overkill, until they want to repurpose the data for tasks requiring survey-grade data. Others would say different workflows because the two are unique through the hardware and software used. But most would say cost of data acquisition. You’ve heard it said: “You pay for accuracy.” Assumptions can be used as excuses to validate collecting data at a lower cost and, in this case, that assumption is as misunderstood as the accuracy of the GIS they defend.
It comes down to knowing what you expect from the data at any future point and not necessarily the current task at hand. Would you use a 2-megapixel camera expecting to get 24-megapixel performance and clarity? This example may be oversimplifying the issue, but just think about it. If you know that in the future you may need a higher resolution image, would you or could you use a 2-megapixel picture expecting to get a 24-megapixel image? You can up the resolution but quality limitations exist. Clients sadly often search out the inappropriate tool for the job because of the up-front cost without understanding the ramifications of their decisions.
I ask, what does it cost to rescan for more accurate uses? The cost-effectiveness of H.R. 4233 “map it once, use it many times” comes to mind. Huge efficiencies occur by combining different departmental needs within an organization and sharing funding between the departments. Data should be collected to the accuracy grade of the highest accuracy-required task, not the lowest. Understanding this need not be a legislative mandate, but it’s a step in the right direction. It starts with a client’s understanding of their end use or possible future use needs, which have to be to be understood before the “tool” or method of collection is chosen. How many times has a client come to you expecting to use an asset-inventory data set and then later expecting, or needing, highway design data? Of course, there is the issue of shelf life in a dynamic world, but I digress.
Underestimating the need is not a new issue. I saw it in the static-scan world supplying architectural needs and evaluating which scan system to use for the intended end result: as-built vs. detailed restoration would equate to a 10 mm noise-level scanner opposed to a 500-micron scanner. With asset inventory vs. highway design, the same evaluation assessment of need should be anticipated. A good practice is to use equipment capable of supplying one grade better than what you are trying to accomplish. Any adjustment to data can introduce error that can affect accuracy, so choose wisely during the planning stages of a project to reduce your time and frustration.
We need to smarten up our clients, not data, regarding data accuracy, how it affects them and how costs can be shared within an organization without dummying down the options.