Line index
A line index traces the route flown by the aircraft. Marks along a flight line represent photograph centers or positions, and labels identify frames or groups of frames. Not every individual frame is always labeled.
What is an aerial photography index?
An aerial photography index is a map or diagram showing where photographs were taken during an aerial survey. The photographs are often called frames. As an aircraft followed one or more flight lines, its camera made a sequence of photographs at regular intervals. Neighboring photographs usually overlap.
The index connects locations on the ground with the identifiers written on the photographs. It is a finding aid, not necessarily a precise map of each photograph’s full coverage. Use it to identify the frames most likely to show your area of interest, then inspect nearby frames as well.
Common types of indexes
Spot index
A spot index places a dot or other symbol at the approximate center of each photograph. The identifier next to a point tells you which frame is associated with that location. A point does not show the photograph’s exact boundary.
Mosaic index
A mosaic index arranges reduced copies of the aerial photographs into a continuous-looking picture of the surveyed area. Photograph edges or printed identifiers help distinguish the overlapping frames.
Some Humboldt County indexes combine features of more than one type, and terminology on the original materials may vary.
How to use an index
- Choose a likely index. Use the date, project symbol, title, and collection information to narrow the results.
- Locate your area of interest. Look for roads, rivers, communities, section lines, or other recognizable features.
- Identify nearby frames. On a line or spot index, note the labels beside the closest points. On a mosaic, look for identifiers printed within or along the photograph edges.
- Record the complete identifier. The index may abbreviate an identifier by omitting a project prefix that is supplied elsewhere. Preserve letters, numbers, and suffixes such as A or B, because they may distinguish separate photographs.
- Check neighboring photographs. Because aerial photographs overlap and index positions are approximate, a nearby frame may provide a better view.
Indexes in the Aerial Photo Finder
When staff have matched photographs to an index and completed human review, the index record displays those photographs as verified related records. Some indexes also include a visual flight-line display. Relationships that have not been human verified are not shown.
An index may depict photographs that have not yet been digitized, described, identified, or matched. The absence of a related photograph in the Finder does not necessarily mean that the photograph never existed or is not held in the physical collection.
Interpreting coverage
- A point generally represents an approximate photograph center, not an exact coverage boundary.
- Flight paths, labels, and map backgrounds may be distorted, faded, incomplete, or difficult to read.
- Dates on an index can describe the index itself, the photographs it represents, or a range of survey dates. Read the record notes and date evidence carefully.
- Project symbols and frame-number formats may vary across agencies and surveys.
Further reading
These explanations were informed by the UCSB Library’s guides to aerial photography and reading aerial photography indexes. The guidance on this page has been adapted for the Humboldt collections.