Screens and Satellites

History of Maps and Cartography

Course 1 · Chapter 9 · Photographs, Satellites and Screens

In a little over a century, mapping moved from men with chains and theodolites to aircraft cameras, satellites and a phone in the pocket that knows where it is. The tools changed more than the questions: someone still chooses what to show, from which data and for whom. This chapter traces the technical steps and then asks who makes today's maps. Names and dates are from general knowledge and flagged "check".

From the Air

Aerial photography

Photographs from balloons and aircraft were used for mapping in the First World War, and after it, overlapping photographs were used to make measured maps by the science of photogrammetry (check).

Stereo pairs

Two overlapping photographs viewed together give a three-dimensional impression, from which heights and contours can be measured (check).

Ground control

Known points on the ground, fixed by the survey network of the previous chapter, tie the photographs to the national grid.

From Space

Step (check each)	Detail
Landsat, from 1972	A US programme of Earth-observing satellites that has provided a long record of images of land cover and change.
Remote sensing	Sensors record light at various wavelengths, which allows maps of vegetation, water and heat, not only of what the eye sees.
GPS	A US system of satellites developed for the military and opened to civilian use. A receiver finds its position by timing signals from several satellites, using the same principle of time as the longitude problem (check).
Other systems	GLONASS (Russia), Galileo (the European Union) and BeiDou (China) provide similar services (check).
Radar and laser	Lidar, a laser scanner, can map the ground surface even under tree cover, and has revealed archaeological sites hidden by forests (check examples).

Time and position A GPS receiver needs an accurate time because it calculates distance from the time a signal takes to arrive. An error of a millionth of a second is a few hundred metres. The satellites carry atomic clocks, and the system must also correct for effects of relativity (check). The old longitude problem is solved by the same principle, with far better clocks.

GIS and Web Maps

GIS. A geographic information system stores data in layers, such as roads, rivers and census areas, which can be combined and analysed. Early work in the 1960s, such as the Canada Geographic Information System, led to the commercial tools of today (check).
Web maps. Services such as Google Maps (launched 2005) and its competitors put zoomable maps into a browser, using the Web Mercator projection (check).
OpenStreetMap. Founded in 2004 in London by Steve Coast, it is an open map built by volunteers, using GPS traces, aerial imagery and local knowledge, and used by many apps (check).
Phones. Navigation apps combine maps, GPS and live traffic data, collected from users' devices.
Open data. Many national mapping agencies, including Ordnance Survey, have released some data free of charge (check which).

Question	Why it still matters (check each)
Who decides what is shown?	A company chooses which places and labels appear at each zoom level.
Whose names?	Disputed territories may be labelled differently for different users.
Who is missing?	Data is thinner where fewer people contribute or where little is recorded.
What is collected about you?	Location history is valuable, and private, data.
What happens when it is wrong?	Drivers have followed routes into hazards and onto unsuitable roads.

A British comparison Ordnance Survey's open data allows anyone to build maps of Britain, and Edinburgh's volunteers have mapped footpaths, cycle routes and shops on OpenStreetMap. Comparing the same street on Ordnance Survey, OpenStreetMap and a commercial service shows three different decisions about what is worth showing (check the layers available).

Check the details Check launch dates, technical claims and the status of services before relying on them, since they change quickly. Treat any screen map as a product of choices by a company or community, not as a neutral window.

Exercises

Exercise 1

Explain how a GPS receiver finds its position, and why it needs several satellites.

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Exercise 2

Compare the same neighbourhood on three different digital maps and list the differences in what is shown and named.

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Exercise 3

Describe two ways in which a digital map could be wrong or misleading, and how a user could detect each.

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Quick Reference

Air: aerial photographs, stereo pairs, ground control
Space: Landsat, GPS and other satellite systems
Data: GIS layers, lidar, open data
Web: Web Mercator, OpenStreetMap
Questions: who chooses, whose names, who is missing