## 1 Introduction  
During the past decade the laser pulse rates of commercially manufactured airborne mapping Light Detection and Ranging (LiDAR) systems, also known as Airborne Laser Scanning (ALS) systems have increased from several thousand pulses per second to hundreds of thousands of pulses per second--by about a factor of 50 to 100 [1]. Most units now record multiple stops (returns) per laser pulse, as well as the intensity (signal strength) of the reflected signal, and some digitize and record the full return signal or waveform. The immediate output from an ALS unit is a point cloud, consisting of the coordinates of unequally spaced points (returns) from which sufficient laser light has been reflected to be detected by the sensor. These \"raw\" point clouds are then processed using various algorithms [2] that may consider both the distribution and intensity of the returns, in order to classify the returns as most likely being from vegetation, human engineered structures, or the 'bare earth.' Or, when the ALS utilizes a laser with a wavelength that penetrates water, points may also be classified as most likely being from the surface, within, or the bottom of water bodies, including streams, lakes and near shore coastal waters [3]. Once all of the recorded returns have been classified, a variety of algorithms may be used to extract and display information sought for a particular application. Often the points identified as ground points are used to produce \"bare earth\" geodetic images (rasters that provide elevation information for an X, Y coordinate array), on which every element on the raster (image) is known in a well-defined geodetic and temporal reference frame. These rasters can also present other types of spectral and spatial information derived from the ALS observations such as return intensity or elevation variation (surface roughness) or return density. The exponential development of ALS technology triggered an explosion in its use for archaeology with early applications in Europe [4, 5, 6, 7, 8, 9] and one target of opportunity in Mesoamerica in 2001 [10]. However the application of the technique gained significant momentum for prospection in regions covered with dense vegetation, including the multi-layer tropical and sub-tropical rain forests of South and Central America [11, 12], as well as South East Asia [13, 14, 15]. The ability to map landscapes hidden beneath dense vegetation is resulting in a revolution in archaeology arguably comparable to that caused by the introduction of radiocarbon dating some decades ago [13]. Also, as more and more work is being done, researchers have been invigorated with its potential and intrigued by its limitations. Questions include: How small an object can be detected [16, 17]? How accurate are the data? Why are some features clearly visible in an area but not as clear in a different area, of the same project? Why do causeways, which are very difficult to see on the ground because of their small elevation relief, appear very clearly in the ALS observations, while other features with larger elevation relief are easy to spot on the ground, but are hard to spot with ALS data? Often researchers compare ALS rasters with satellite or airborne imagery, and try to use analogies to compare and understand the information they contain. While there are some parallels, the datasets are more different than alike. Spaceborne or airborne imagery is generally highly standardized, and there is extensive documentation addressing all aspects of the collection and processing of the raw data into imagery products. On the other hand ALS data products and the procedures used to collect, reduce, and process the data into final deliverables are not standardized. Not all ALS products are created equal, nor should they be. The considerations and decisions made at each of the ALS dataworkflow steps affect the quality of the final products and hence their usefulness in answering specific research questions. It is important for the researcher to understand these decisions and options.