methods of data development
A multitude of methods exist to record system data from pencil drawings to digital CAD drawings. The five primary methods that have been investigated in the process of data collection by Kansas Rural Water are:
heads up digitizing over scanned engineering drawings,
heads up digitizing over digital orthophotography with engineering drawing reference,
automated raster/vector conversion,
and field data collection.
Manual digitizing with a digitizing tablet has been widely used in the industry. With this method, the operator manually traces all the lines from his hardcopy map using a pointer device and creates an identical digital map on his computer. For example, collecting a series of points along the line digitizes the line. Although this method is straight forward, it requires an experienced operator and is very time consuming. For a complex contour map, it can take a person many days to get the map fully digitized.
Another major drawback of this method is its low accuracy. The accuracy of manual digitizing merely depends on how accurate the hardcopy map is duplicated on a computer by hand. The spatial accuracy level the human hand can resolve is about 40 DPI (dots per inch) in the best case and will be lower while the operator is tired and bored after working on it for a period of time.
Heads-up Digitizing Over Scanned Engineering Drawings
Heads-up digitizing is similar to manual digitizing in the way the lines have to be traced by hand, but it works directly on the computer screen using the scanned raster image (engineering drawing) as backdrop.
While lines are still manually traced, the accuracy level is greater than using digitizing tablet because the raster images are scanned at high resolution, normally from 200 dots per inch (DPI) to 1600 DPI. With the help of the display tools, such as zoom in and out, the operator can actually work with the resolution of the raster data therefore digitize at a higher accuracy level. However, the accuracy level is still not guaranteed because it is highly dependent on the operator and how he digitizes. This method is also time-consuming and takes about same amount of time as the manual digitizing method.
Heads-up Digitizing Over Digital Orthophotography with Engineering Drawing Reference
Heads-up digitizing in this method of data development is similar to the previous method, heads up digitizing over scanned engineering drawings, except that it is done over digital orthophotography with engineering drawings used as a reference.
Automated Raster/Vector Conversion of Scanned Engineering Drawings
Automatic digitizing, or so called automated raster to vector conversion, traces lines automatically from the scanned raster image using image processing and pattern recognition techniques. The idea behind automated raster to vector conversion algorithm is to let the computer do the actual line tracing and eliminate manual tracing performed by the operator.
The automation of the raster to vector conversion process has been a major research focus during the past two decades. Only in recent years has automated raster to vector conversion software on PCs and small computers become practical and commercially available for data acquisition applications. However, fully automatic raster to vector conversion cannot be applied in cases such as low image quality and complex layers.
Field Data Collection
Field data collection is a process of data development where the coordinates of visible system features are collected. The accuracy of the collection equipment is paramount when the features have to be later re-located, such as buried valves. This process in itself, although accurate, does not lend itself readily to map production. An auxiliary system is required in order to make the data more "user friendly".
The Winning Combination
Kansas Rural Water has determined that the best process for data development is a combination of two processes, heads up digitizing over digital orthophotography with engineering drawing reference augmented with field data collection.
The data from the field collection are imported over the digital orthophotography and if the scales and projections are correct, the points appear on the aerial just where they are in the real world. The heads up digitizing begins with the placement of non-visible system features, those that could not be field collected, over the aerial. Referring to the engineering drawing aids in the correct placement of features, such as line location, and the correct layout of interchanges, such as valve intersections.
The field data collection is crucial in that it shows the exact placement of a feature versus a close approximation through the digitizing process.