From the Ground Up: Selecting the right tool from the toolbox.
Mapping firms have a lot of tools in their toolboxes today. Technology has played an important role in adding new tools and improving the ones we have been working with for years. Moreover, technology continues to change the way we use these tools, affecting the decision making process for selecting the right tool for the job.
The creation of Digital Terrain Models (DTMs) is a great example of an area where many tools are available to the mapping professional. While the available tools are all similar, there are some profound differences in them as well. A careful evaluation of accuracy, project size, land cover and terrain on a project-by-project basis is necessary before making the selection.
Comparison of the ToolsA prospective client called our office last year and specifically requested LIDAR DTM mapping for his project site. I began collecting the information needed to provide him with a project estimate during our telephone call. I immediately knew we were headed down the wrong path when I found out the area of the site was only 10 acres. Conventional photography and mapping would be much cheaper. But even that approach would have been overkill. A ground survey with a total station or RTK GPS receiver was the best tool for this job. Therefore I recommended two surveying firms with offices near the project site and felt comfortable with the methods that would be used to provide the mapping he needed.
All of the tools in our toolbox have individual strengths and weaknesses. LIDAR is a great tool, but it is not well-suited for small project sites because of the high fixed costs (mobilization, calibration, etc.) associated with the technology. Change the area in this example from 10 to 10,000 acres and we most likely have an ideal match.
And to further complicate the issue, there are several additional combinations within each of these options. For example, conventional ground surveys could employ a total station, an RTK GPS receiver, or a ground laser scanner to produce the mapping. Similar analogies can be found in the other options listed.
Conventional ground surveys can provide a high degree of accuracy for creating a DTM, but as a general rule they are only appropriate for smaller sites due to cost issues. Moreover, other considerations can come into play. For example, conventional surveys may not be appropriate for creating a DTM of a heavily traveled roadway due to safety and maintenance of traffic considerations. Access to all areas within the site is required for the field surveying; this factor can eliminate this approach for some projects.
Conventional mapping using a traditional fixed-wing (airplane) platform for photo collection is the most used tool in the toolbox. It has application in many diverse areas and is flexible in terms of price. We complete hundreds of projects each year ranging from a few acres to several thousand acres in this manner. Furthermore, you can map many sites without ever stepping foot on private land, which can be very important for some projects. But this approach also has its limitations. Often it’s not the right tool for areas with dense trees or agricultural fields, unless the photography can be captured in the late fall or early spring when vegetation is at a minimum. And it may not prove accurate enough for the most demanding of projects. Luckily the toolbox is filled with other tools that are appropriate alternatives.
Precision mapping using a rotary-wing (helicopter) platform for photo collection can provide extremely accurate mapping. The ability to collect the photography at an elevation much lower than possible with an airplane opens the door for very high accuracies. The cost of the photo acquisition is higher due to the greater expense of operating a helicopter. The cost of setting the necessary photo control is usually much greater as the accuracy requirements are much higher than what is required on a normal photogrammetry project. Also, more control points are required due to the increased number of photos. This type of precision mapping is gaining a lot of popularity in transportation mapping of existing facilities, as it provides a safe and efficient way of mapping high-traffic roadways.
Finally, LIDAR can be a tremendous tool for the right project. LIDAR is characterized by a high fixed and low variable cost. This means it generally isn’t used for small projects but can prove to be very cost effective for larger ones. LIDAR can also penetrate vegetation to some degree and therefore can provide DTMs when other photogrammetric methods are rendered ineffective. On the downside, there is a limit to the accuracy that can be attained using LIDAR. Furthermore, the technology is still evolving and the high initial cost of the LIDAR units serves as a barrier to entry for many firms. The way this tool is applied to projects will continue to change at a rapid pace over the next couple of years.
The most common methods used in creating DTM mapping include:
- Conventional ground surveys
- Conventional aerial photography and mapping
- Fixed Wing (helicopter) aerial photography and mapping
ConclusionThe tools available to mapping professionals are very impressive. The toolbox looks much different today than it did just a few years ago. But I think we can make the same statement about the tools available to every surveying, mapping and GIS services firm around the country. The new look is certainly an improvement. I think we all know we’ll continue to see new tools emerge for our use in the near future. And that’s good for us all.