The time investment in implementing the mobile GIS approach is still considerable, as it involved both pre-fieldwork and post-fieldwork processing steps. Pre-fieldwork tasks, discussed above, include acquiring and preparing regional datasets, and designing digital forms that are appropriate for the project. Post-fieldwork processing involved standard issues such as downloading all data to a laptop from various devices, tagging folders of digital photos with the associated ArchID, GPS post-processing, as well as analytical processing steps such as deriving meaningful indices from the digital data. Post-fieldwork tasks also involved some unanticipated and time consuming labor, such as cleaning inconsistent datasets to prepare them for general analysis, and other management issues. These inconsistent data include the records gathered during two periods when the system was not functioning smoothly as described in Table 5-7.

Table 5-7. Sources of inconsistent data during the 2003 project, and means of avoiding these problems in future projects.

Some of the processing steps are the result of employing GPS based mapping, and are therefore largely inevitable. Despite post-processing, the polygons and polylines gathered using GPS in Streaming mode contained a lot of redundancy and required geometry validation. These redundant positions were especially abundant where the person mapping a feature had to slow down or stop in the process of delimiting the feature. In these cases a number of vertices would be gathered as a small cloud (within the positional error of the GPS) and this resulted in a line intersection and short segments. Processing the data to resolve these GPS derived problems was accomplished in ArcGIS 9 using the following processing steps

(1) The "Repair Geometry" function was used to resolve intersections in a single polygon or polyline.

(2) Polygons were converted to polylines and the ArcToolbox > Data Management > Features > Simplify Line operation with a 10 cm tolerance was applied the data. The 10cm range for simplifying the lines reflects the data quality. In this case, the post-processed accuracy of the spatial data was 1-2m as stated by Trimble Pathfinder Office.

More recent versions of Arcpad (v7) and other mobile GIS software such as Terrasync provide a movement filter for datalogging that allows the user to specify a minimum distance setting, so that data are not logged unless one continues moving. For example, during streaming mode in 2003 the GPS was logging a vertex from the average of every 2 positions for a smoother line (at a 0.5 second rate this resulted in a vertex every second). A feature of the newer Arcpad 7 allows the user to specify that a vertex should be logged only after 3m of movement.

This option would have resulted in cleaner data during the 2003 season, although with low real-time accuracy the movement filter is relatively coarse and it is probably accurate to only within 10-15m. In North America and Europe, where SBAS (WAAS or EGNOS) correction signals are available, real-time positions are approximately 5-10m and these movement filters for datalogging function adequately on low-cost receivers. However in South America SBAS correction will not be available in the foreseeable future and movement filters are probably most relevant for faster GPS mapping tasks such as mapping from a moving vehicle. It is conceivable that GPS movement filters will actually result in noisier data because exceptionally large position measurements are logged, but the majority of positions are filtered out because the distance of movement is not sufficient.