3.3.1. Burned Area Estimates
 When using long-term data sets to analyze the impacts of fire on carbon cycling, it is desirable to develop uncertainty values for the burned area estimates and other parameters used in models to estimate carbon flux and storage [see, e.g., French et al., 2004]. In this section, we discuss factors that control uncertainties in burned area estimates that are available for modeling the impacts of fire on carbon cycling in North American forests.
 Over the past 130 years since scientists and managers began compiling data to estimate burned area in North America, the methods used for mapping fires have continuously evolved in response to advances in technology. To understand the influence of technology, consider the three factors required to estimate burned area for large fire events (which provides a foundation for estimating burned area over large regions). First, access to the areas where fires occur is needed. Second, there must be a means to observe the entire fire perimeter. And third, there must be a way to accurately locate and map the position of the fire perimeter. Below, we will discuss how advances in technology have provided the means to address the above information requirements within the United States. Similar arguments could be made for Canada and Mexico based upon the particular set of circumstances for these countries.
 The first technological development that addressed these information needs was the creation of a surface transportation network across the U.S. This included the development of railroads in the late 1800s and early 1900s, the continuous expansion of road networks into remote areas throughout the 20th century, and the rapid expansion of the automotive industry that provided the basis for using the expanded road network to conduct fire reconnaissance. These developments all provided an ever-expanding ability to access a larger portion of remote areas where wildland fires were common, especially in the western U.S.
 In spite of these developments, there was still a considerable area in the U.S. that could not be accessed by roads, especially in Alaska. These limitations were initially overcome by the development of the aviation industry, in particular over the last half of the 20th century. While aircraft were available for observing fires in remote regions as early as the 1920s, aviation resources were not incorporated into fire management agencies until the late 1940s, when surplus aircraft and a trained pilot force became available at the end of World War II. The ability to map fires from satellite imagery beginning in the mid-1970s overcame the need for any sort of ground or airborne transportation to access areas where fires occurred.
 Once access is gained to areas where fires occur, there must be some means to observe the perimeters for individual fire events. For large fire events, precise mapping of fire perimeters is not feasible using ground transportation alone, and requires some sort of elevated observation point (e.g., a mountain top, fire tower, or aerial platform). The means to adequately address this requirement was not available in the U.S. until the late 1940s, when fire management agencies began to implement capabilities for the aerial monitoring of fires in a systematic fashion. Again, the availability of satellite remote sensing imagery beginning in the 1970s provided a solution to address this issue as well.
 Finally, once the means became available to gain access to and observe the perimeter of the fire event, the ability to accurately locate the perimeter of the fire event was needed. While USGS produced its first map of the United States in 1879, maps at a scale needed for locating large fire events were not available across the entire U.S. until the 1970s. Prior to the production of these maps, fire managers were not only required to produce maps of fire perimeters, but also the baseline maps of the areas where the fires occurred, including information on location (latitude and longitudes of the region) and the locations of the prominent cartographic features of the region (e.g., roads, streams, rivers, lakes, etc.) [see, e.g., Kasischke et al., 2002, Figure 2]. Thus, the accuracy of the fire perimeter maps in terms of estimating burned area depended upon the cartographic skills of the observer, even when baseline maps became widely available. This mapping limitation was to some extent overcome by the availability of satellite remote sensing imagery beginning in the mid-1970s, if the fire management agency had the resources to obtain and process the satellite data. The development of global positioning systems (GPS) in the early 1990s provided a convenient means for locating the perimeters of fire events. Using the data from GPS within a GIS, maps of fire perimeters can easily be generated, as can highly accurate estimates of burned area.
 As discussed earlier, the use of satellite imagery in many cases provides the optimal solution for addressing all the requirements for mapping burn perimeters and estimating burned areas. Even though remote sensing data suitable for mapping burned area over large areas has been available since the mid 1970s, it has been only recently that programs have been developed to exploit this technology. In particular, in the U.S., the Monitoring Trends in Burn Severity (MTBS) project is in the process of generating a perimeter map for all fire events larger than 400 ha in size across the U.S. for the years 1984–2010. In many instances, fire managers now use satellite data as the primary means for mapping perimeters of fire events.
 Why hasn't satellite remote sensing imagery been adapted as the primary means for fire management agencies to map fire perimeters? The answer to this question is complex. First, the in-house capabilities within land management agencies to process and analyze geospatial data have slowly evolved over the past two decades. It has only been during the late 2000s that the infrastructure (both technology and human resources) needed to routinely process and analyze satellite been implemented across all agencies responsible for fire management. Second, there are issues related to the production of timely and/or redundant information. Geospatial technicians are now often called upon to produce continuous updates of fire perimeter maps based on using observations made with GPS collected from aerial platforms. It is not uncommon to produce updates on a daily basis, especially for large, active fire events. This information requirement cannot be fulfilled using medium resolution satellite data because of the repeat frequency of these satellites. Because of this practice, the regeneration of a fire perimeter map using satellite data collected at the end of a fire season is often viewed as producing redundant information. Thus, the use of satellite data for mapping fire perimeters over large areas only occurs when fire management agencies do not have the resources to monitor fires in remote regions [see, e.g., Epp and Lanoville, 1996].
 The evolution of technology over the past century has played a central role in improving the accuracy of estimates of burned area in the United States and Canada, yet few studies have assigned error bounds to historical burned area estimates [see, e.g., Kasischke et al., 2002]. For example, what is the error or uncertainty bound on the estimate of 22 million ha burned in the early 1930s for the U.S. that is reported in the annual reports of Wildland Fire Statistics (USDA) given the restrictions placed on mapping fires based on the available technology of that era?
 To examine this uncertainty issue, a simple thought exercise was carried out on how the development of various technologies has influenced relative uncertainties in estimating burned area. Uncertainty was rated on a relative scale of 0 to 1/3 for three different categories: fire access, perimeter observation, and perimeter mapping. The effects of five different technological advances were evaluated in terms of the reliability of burn estimates in the 3 categories: development of a ground transportation network; development of air transportation; production of USGS maps; GPS development; and production of satellite maps.
 The results from this assessment are presented in Figure 7, which indicates there was likely very high uncertainty in burned area estimates prior to 1950, and that the development of technologies has gradually reduced uncertainties over time. Note that this assessment is very rudimentary in that it is based upon an incomplete understanding of the practices used for mapping fires during years prior to 1950, and on best guesses as to how advances in technologies affected the uncertainties in the three areas. It very well could be that the weighting for the uncertainty categories might be different than those used to generate Figure 7. For example, perimeter observation and mapping may play a greater role in uncertainties than fire access. Regardless of the assumptions used, we believe the basic trend in Figure 7 to be consistent with reality, with very high levels of uncertainties existing prior to the 1920s, and a continuous decrease in uncertainties due to the implementation of different technologies over time.
Figure 7. Patterns of relative uncertainty in estimating burned area over time in the United States based on advances in mapping technologies.
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3.3.2. Seasonal Fire Activity
 Care must also be taken in using other fire information reported by management agencies. While some have attempted to use fire management records to estimate changes in fire season-length [see, e.g., Westerling et al., 2006], this approach may be problematic. In particular, there are often two criteria used to designate a fire as being out. The first designation for an end date is based on the physical characteristics of a fire, e.g., when does the fire perimeter stop growing, or when does smoldering combustion of fuels that might provide the basis for further fire spread end. The second designation for an end date is based on administrative considerations. The logistics for fire management activities are often accounted for on an individual fire basis. In the U.S., this approach was developed for several reasons. First, areas that are burning in large fire events often fall under the jurisdiction of different land management agencies that have individual budgets and manpower for fire management. Establishing a single financial accounting system for each fire event provides the basis for sharing of resources between agencies in managing large fire events, as well as the flexibility to shift resources between fire events as circumstances dictate. Second, this approach allows for sharing of resources between fire management agencies, especially the shifting of resources from areas with low fire activity to areas with high fire activity. During large fire years especially, individual fire events are often not declared out until late in the fire season because of limited resources for actually determining whether a fire event has physically ended and the need for flexibility to administratively assign resources to an event should it become active again. As an example, during the large fire season of 2004 in Alaska, daily fire reports for individual events as well as daily observations of fire hot spots from thermal IR satellite remote sensing systems showed that fire activity on the majority of large fire events ended in early September. Yet for management purposes, the out dates for a large number of fire events were in mid to late October. In some cases, fires declared out often continue to smolder and become the source of fire ignitions during the following fire seasons. This was the case in Alaska for several fires that started in May of 2010.