One of the challenges faced when using traditional land surveying methods is the necessity of surveying areas that are covered by thick vegetation. Some surveying projects require the measurement of such vegetation, while most applications require measurement of the terrain itself, rather than the vegetation covering it. Topographic surveys may or may not require the measurement of vegetation, depending on their intended purpose.
When on the ground, land surveyors may find that traditional surveying equipment is blocked by large trees or other obstructions. They may even be unable to traverse the land because of steep hills, inconvenient streams, or other natural or man-made features. Combined, these features may make surveying difficult or even impossible on the land itself, particularly in undeveloped areas. However, there are ways around these obstructions that allow surveyors to create accurate and precise surveys.
Some methods of surveying can penetrate trees and ground cover. Among these is LiDAR, a laser-based method. Because the laser device is flown over the area in an airplane, it is ideal for heavily forested areas where access on foot may be difficult or impossible.
Other surveying applications, on the other hand, require measurement of the actual vegetation. Similar aerial methods do not penetrate treetops, and therefore can give an idea of the vegetation while still allowing the surveyor to work from a distance.
One type of surveying, known as “vegetation surveying,” is particularly interested in the vegetation found in a given area. Unlike standard land surveying, vegetation surveying often depicts rough boundaries rather than strict lines. Vegetation surveying, or the mapping of plant habitats, is a valuable tool for botanists, environmentalists, and other earth science applications. Depending upon its intended use, a vegetation survey may indicate areas with vegetation and those without, or the species of plant and their density and location. These maps may be used to identify sensitive ecological areas (such as wetlands), map the spread of plants, or examine environmental changes following natural or man-made events.
In cases where vegetation surveying is desirable, measurements may be taken using aerial methods or using a surveyor’s transit or total station to measure vegetation height and to lay out a grid of the terrain, onto which vegetation can be measured. The same grid can then be used by another surveying team after a period of time to measure changes in vegetation and terrain.
While topographical maps may display vegetation or man-made features, a digital terrain model generally only represents the ground topography and terrain underneath the vegetation. Digital terrain models may also be referred to as bare-earth models, while digital surface models include above-ground features such as vegetation.
There are a variety of surveying methods used to create topographic surveys or digital terrain models, such as direct surveying (with a surveyor’s transit or total station) or remote sensing technologies such as aerial and satellite imagery, LiDAR methods, and photogrammery. The most appropriate method depends on the area being surveyed and the amount and type of data required. Some surveying methods used to create these models, such as radar, reflect the highest elevation point on a given location, whether this is the top of a tree or building or bare ground, while others are intended for the measurement of the terrain itself.