Introduction

Remote sensing—aerial photography in particular—provides perhaps the best means to assess landcover changes over time. This is especially true when assessing changes over long periods of time. Aerial photography provides an invaluable record of land use and landcover conditions—frequently dating back to the 1930s. No other source of remotely sensed imagery is available prior to the 1970s. Most national forests have repeat photography on approximately a 10-year cycle. This rich photo record can indicate changing threat conditions through time, including threats such as insect damage, fuels buildup, unmanaged off-highway vehicle (OHV) use, and loss of open space.

We’ve long recognized and valued the quality information content of photos; however, we’ve also long struggled with how to get our photos into our Geographical Information Systems (GIS). Current technologies now allow Forest Service employees to input photos into a GIS accurately and easily. Within the GIS, we can evaluate, analyze, and document important changes.

This analysis discusses selected photo characteristics, available software solutions, and techniques for historical photo orthocorrection—along with examples using historical imagery to document selected changing threat levels over time.

The methods described in this paper are available throughout the Forest Service and provide the most consistent and objective method for detecting, assessing, and monitoring landcover changes through time.