From the Ground Up-Reflections after a flight season.
August 1, 2007
As I am writing this, the traditional flying season for mapping projects has just ended. Most photo acquisition takes place from late autumn, when the leaves fall off deciduous trees, to early spring, when the leaves reappear.
There is generally a period within that time frame centered about the winter solstice when the sun is at its lowest point in the sky, and no flying takes place due to the long shadows created by the low sun angles. Flying season is an intense time; a lot of photography must be captured in a short period of time to feed the office production for the coming months and to meet the needs of clients.
Attention to detail is critically important to success during this time because very few projects are all that similar these days--different from our operations just a few short years ago. Today, LiDAR is used as the basis of elevation models for some projects. Traditional photogrammetry forms the basis of others. Large-format digital cameras have moved to the forefront of imagery capture, yet film cameras continue to produce highly-accurate, quality imagery for many traditional mapping projects. Most projects are flown in fixed wing aircraft, but high-accuracy projects may demand low-altitude capture from a helicopter platform to achieve accuracy requirements. Many clients want multiple image types; therefore, requests for both true color and color infrared orthophotos on a single project are increasing.
One of the biggest changes we have seen in the mapping business over the last few years is the way we control projects. More than half of our projects require the placement of ground targets prior to flight. Just about all large projects make use of airborne GPS (AGPS) collection during the flight to reduce the number of required control points. Others make use of inertial technology to control the flight entirely from the air. But for inertial projects, and certainly for LiDAR elevation model collections, it is critically important to have quality control check points on the ground to verify the accuracy of the data we provide as professionals. Therefore, some type of ground surveying effort is almost always required for mapping projects.
As I look back on the last six months (after catching my breath), I think we have had our most successful flight season in terms of working with professional surveyors around the country. We received critical support in terms of targeting and ground control; logistical support during flight operations; and quality control efforts for our projects. Were these surveying professionals more knowledgeable and better equipped to provide the support we needed? Did we do a better job of communicating our needs and managing these services? Was technology a major factor in this success? Actually, I believe that all three contributed to this success during the flying season.
Project DiversityThe diversity of the projects that we take on during a typical flying season never ceases to amaze me. We have used low-altitude beach photography during a holiday weekend to get a head count on the number of people at a beach. The ultimate client used this information for planning in terms of access, parking, restroom and related facilities. We have collected mid-summer, leaf-on LiDAR elevation data to analyze the obstructions in the approach and departure zones for new runways at a major international airport. We have flown 3,000 feet below the rim of the Grand Canyon to gain high-resolution mapping of the Colorado River for environmental monitoring. Low-altitude helicopter-based mapping has been used to analyze the superelevation on roadways that experienced higher than normal accidents during rain events. And of course, we have completed a steady diet of what I consider more traditional mapping applications for new roadways, land development or local governments. Local surveying professionals have played a major role in the success of each of these projects.
Our relationship with professional surveyors around the country is a two-way street. We perform hundreds of projects each year for professional surveyors where they are the prime professional. We have several hundred other projects where we reach out to local surveyors to support projects when we are the lead professional. This is nothing new; we have enjoyed this healthy two-way relationship for many years. What is new is the diversity of ways we create the data, and consequently the diversity in the methods employed to acquire information on the ground.
Changing TechnologiesFifteen years ago, establishing ground control for mapping projects almost always meant running a traverse through the control points with a total station to determine the horizontal positions and extending differential level loops to establish the elevations. When GPS technology became popular, it resulted in significant changes and created considerable cost efficiencies and sizable improvements in the horizontal accuracies that could be achieved in these efforts. In the early days of GPS, most control surveys were completed with three- or four-person crews using static network techniques. While this was a considerable improvement over conventional surveying methods, the size of the crew and number and length of observations kept the unit costs per control point relatively high for these operations.
Real-time Kinematic (RTK) and code-phase differential observations brought new options to the field. For the right project, these methods produce significant cost savings. RTK control surveys, typically completed with a two-person crew, can produce accuracies that approach those of static or rapid-static observations. The cost savings of RTK versus static observations can be considerable, especially when the project area is relatively small or in areas where RTK networks are available. Ideally, each control point will have two independent occupations under varying GPS satellite configurations, and will be processed against independent base stations to provide confidence in the resulting positions.
Code-phase differential, or submeter GPS, observations provide more options and cost efficiencies. Typical horizontal accuracies are around 2.5 feet with vertical accuracies of 5 feet for these observations. They can be accomplished with a single person crew and obtain observations ranging from a few seconds to a few minutes. These accuracies aren’t right for every project, but for those projects requiring lesser accuracies, the cost savings can be substantial.
Applications like OPUS-RS (see “From the Ground Up,” POB May 2007), however, have in many ways rendered code-phase surveys obsolete, at least in the realm of ground control for mapping applications. The accuracies available through OPUS-RS are far superior to those possible with code-phase techniques; the National Geodetic Survey (NGS) predicts 5 cm horizontal and 10 cm vertical accuracies at the 92 percent confidence interval for 15-minute observations posted to the service. Yes, 15 minutes is more time than required in submeter applications. But by the time a GPS technician snaps a couple photographs of the occupation and draws a sketch of the control point and area immediately surrounding the point (typical procedures for photo control surveys), 15 minutes have elapsed; in that time, the OPUS-RS service has collected enough data for reliable positioning of the point. Our tests and the surveys completed for us by many subconsultants around the country have verified these accuracy expectations for the OPUS-RS service.
Significant improvements extend beyond the hardware, software and methods used in doing the actual surveying in the field. A quick scan of online imagery from services like Google Earth can be used to gain a handle on project site conditions before putting together a cost estimate or project approach. Similarly, like sources can be used to scout control point locations prior to mobilizing to the field. Both capabilities can be significant advantages in planning for field observations. Moreover, products like Delorme Street Atlas can be very useful in putting together maps and ASCII files of general coordinate locations where control points are needed, greatly improving the efficiency of those in the field. And these locations can easily be uploaded into a handheld GPS device for further improved navigation by field crewmembers during the control layout process.