Did You Know?

No-one knows the exact date when the first map was created. However, we do know that down through the centuries, maps have been written in all sorts of ways and on all sorts of materials. Below are some facts about our maps of the Past.

- Maps were written in all sorts of ways and on all sorts of material. Clickhere to find out more.

- In 1973 two maps from the second century B.C. were found in China. One of these was a topographic map, the other was military. They were both written on silk. Do you think it would be difficult to draw a map on silk?

- Eskimos used driftwood, bone and pebbles to represent routes or trading grounds.

- Early maps were often local, showing people where things were within a limited area.

- In the past, maps were often used by abbeys and also the estates of wealthy landowners.

- Some of the first detailed maps of places were made by armies.

The Manufacturing Process

The production of an accurate topographic map is a long and complex process that may take as much as five years from start to finish. It takes a skilled team of surveyors, engravers, fact checkers, printers, and others to produce a good map. Here is a typical sequence of operations used by the U.S. Geological Survey to produce a 7.5 minute quadrangle topographic map.

Photographing the area

• 1 The area to be mapped must first be photographed from the air. Each section of ground is photographed from two different angles to provide a stereoscopic three-dimensional image that can be converted into contour lines. The sky must be clear, and the sun must be at the proper angle for the type of terrain being photographed. Seasonal

Each section of ground is photographed from two different angles to provide a stereoscopic three-dimensional image that can be converted into contour lines.

factors must also be taken into consideration. For example, in areas where there are deciduous trees, the photos are usually taken between late fall and early spring when the trees are bare and the underlying ground features are more visible.

• 2 The aircraft is flown over the area at a constant altitude in a north-south direction along carefully determined flight paths while special cameras take 10 precisely positioned photographs of each quadrangle. Each camera can cost $250,000 or more.

Surveying the control points

• 3 To ensure the accuracy of a map, the exact location of various control points must be established by field surveys. Typical control points may be the intersection of two roads or other prominent features within the map area. Horizontal control points are surveyed to determine the longitude and latitude, while vertical control points are surveyed separately to determine elevations. The location and elevation of these control points help the map makers correctly position the aerial photo images and assign values to the contour lines.

• 4 While the surveyors are in the field, they also look for features, which may require further checking, such as roads or streams hidden beneath overhanging foliage, or buildings that may have been constructed or demolished since the aerial photographs were taken.