GIAA Mailbag: On sighting distances with digital levels and the accuracy of GPS receivers with WAAS capabilities.
Q: We've been using digital levels successfully at our company. We get good results, and in general, are providing a better service to our clients because of them. The improvements in accuracy and speed are obtained without increasing (and in fact, by decreasing) the times where we have to redo something because an error is discovered in the office. My question is on sighting distances; since the rod is read electronically, can we improve our speed by increasing the distance between setups to perhaps twice the maximum distance of 120 m that we try to achieve when the terrain allows it? Or will this impact accuracy?
A: The digital level is an extremely efficient way to maximize accuracy and productivity. As with all geomatics technology, "stretching" its limits must only be done with an understanding of how it works and how changing one or more of the variables in the process affects the results. In this case, we assume that the 120 m you have between setups implies that you have a maximum sighting distance from the level to the rod of 60 m. It is more than likely that you are at the maximum distance recommended by your instrument manufacturer. As you increase the distance even more, there is the possibility that your instrument will not be able to read the rod. But it is also possible that if it can read the rod that the observations it will make will either take longer, get a less accurate reading than you are accustomed to getting, or both. It is likely that you understand how and why increasing sighting distances when using levels that are read manually causes results to be degraded. Digital levels are affected similarly, although for somewhat different reasons so it is not recommended that you increase sighting distances beyond what the manufacturer recommends. We suggest for improvements in productivity that you investigate opportunities through training your crews, developing strategies for optimal locations of level and turning points, and considering the possibility of using an additional crewmember equipped with another rod.
Q: I have been told by friends in the business that the new GPS receivers intended for surveying that have WAAS capabilities make positioning more accurate. Is this true?
A: The Wide Area Augmentation System (WAAS) was developed to assist aircraft primarily with their navigation functions. It is true that when the WAAS signal is available, the GPS receiver equipped with this capability will be able to provide a more accurate position fix autonomously than without. However, it doesn't improve the intrinsic carrier phase resolving and processing features of the GPS receiver. You may see improvements in positions where you have to estimate the position of the reference receiver by using a WAAS-improved autonomous position. You will also see improvements when using the receiver autonomously for other activities such as reconnaissance or monument search. Remember that because of the location of the WAAS signal emission from a geosynchronous satellite (located in an equatorial orbit and thus at mid-latitudes in the United States, low in vertical angle), that you must be mindful of the potential from signal blockage that may prevent you from taking advantage of the WAAS-enhanced positions.