Comparing historic photographs with modern ones allows us to describe changes in the environment. The earliest photographs available were taken in 1947, after General Patton's manuvers in the desert but before a renewal of activity at Fort Irwin. Several times of repeat photography are available. 1983 photographs show significant changes, mostly as widening of alluvial washes and more bright-colored dust and sand deposited northeast of the Main Post and Bicycle Lake.
Showing changes in the landscape and land use
The images below represent the main operations base of Fort Irwin and Bicycle Lake (a dry lake or playa) at two different times, 1992 and 1947. Then digital images are constructed with 1-meter resolution and are geographically rectified; that is, they are stretched to match the topographic map (the southeast quarter of the Fort Irwin 7.5-minute quadrangle).
The DOQs illustrate many changes in the area over the 45-year span between images. The images can't be compared directly because of the different photographic techniques used. The 1947 image is from photographs, that were scanned and merged, whereas the 1992 image is from digital data- a single band of an infrared data set. In addition, differing sun angles and vegetation differences due to different seasons contribute to the differences in the images.
What can we see that has changed?
Historical digital orthophotographs can allow precise analysis of environmental change. USGS is now constructing digital orthophotographs for much of the Mojave Desert with modern images. It also has constructed pairs of images, from modern and 1947 photographs, of other parts of Fort Irwin and Joshua Tree National Park. These pairs can be used to quantify changes over nearly 50 years.
Geologists have noted for at least a decade that tortoise populations
seem to be much more robust in areas underlain by weathered granite and
its erosional products of sand and gravel. In fact, specific kinds
of granite appear to be favored, kinds that can be easily distinguished
in the field and by remoting sensing, using processed satellite imagery.
Intensive studies in a tortoise preserve along the south side of Fort Irwin are yielding information on this interesting creature and its habitat.