Science & Technology in Action

8th Edition

So where are you? – Mapping Ireland

Ordnance Survey Ireland

Accurate mapping was never so important or so possible as it is today. This lesson outlines the elements of cartography, mapping instruments, surveying, projection, grid reference system, the geoid, map scales and map products.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Accurate mapping was never as important or feasible as it is today. This lesson explores some of the underlying issues, assumptions, developments and uses of maps today. It covers elements of cartography, mapping instruments, surveying, projection, grid reference system, the geoid, map scales and map products.

Cartography
Cartography is the study and practice of mapmaking. As early as 3800 B.C. the Babylonians conducted land surveys for taxation purposes, followed by the Egyptians in order to site, level, and erect the pyramids. Egyptians used measuring ropes, plumb bobs (pointed weights on a string that establishes a vertical line), sighting instruments (in their case a palm branch with a sight-slit in the broader end) and levelling instruments. The Greeks used triangulation techniques and developed the astrolabe (by incorporating a star chart and used by sighting on a star) and the quadrant (which, by means of a plumb bob and a sighting of the North Star, was used to find latitude). Today’s sighting instrument is called a theodolite.

Mapping Ireland
In 1846 the Ordnance Survey Office surveyed the entire island of Ireland for land valuation and taxation purposes. Ireland was the first country in the world to be entirely mapped at a scale as detailed as 6 inches to 1 mile, i.e. 1:10,560. It was completed using triangulation, which is the process of determining the location of a point by measuring angles to it, from each end of a fixed baseline. The surveyors, Colby and Drummond, introduced their own inventions. They made sightings over 67 miles, longer than previously possible, using limelight, and a heliostat (a mirror that turns in time with the Sun so as to keep reflecting sunlight toward a fixed point). They used a 10,372 m baseline on the eastern shore of Lough Foyle and used brass-iron compensation bars to allow for temperature changes. In 1960 the Ordnance Survey of Northern Ireland re-measured the base line using electronic equipment; the difference was approximately 2.5 cm.

Quiz questions

  1. A “plumb bob” is a spherical weight suspended on a line. false
  2. A topological map accurately reflects geographic details. false
  3. The orientation of a map will tell you which side is North. true
  4. A quadrant is a four sided figure. false
  5. A baseline is a fixed line from which other measurements are derived. true
  6. Drummond’s compensation bars were made of brass and iron. true
  7. The earth is a perfect sphere. false
  8. Projection is the representation of a spherical body on a flat surface. true
  9. Transverse projection uses an East-West axis. true
  10. Transverse projection uses a North-South central meridian. true
  11. The origin of the Irish grid is Poolbeg in Dublin. false
  12. LiDAR scanning produces a point cloud. true
  13. Ireland uses its own grid system to demonstrate independence from Britain. false
  14. G0305 means ‘square G, 3 km east, 5 km north’. true
  15. Mean sea level is the same all around Ireland. false
  16. An ellipsoid is an exact local representation of the geoid. false
  17. A map of scale 1:600,000 is a small scale map. true

Glossary of terms

Astrolabe
baseline
cartography
central meridian
compensation bar
datum
digital mapping
eastings and northings
ellipsoid
false origin
geoid
grid
heliostat
Irish grid reference system
land register
limelight
Mean Sea Level
Mercator projection
orientation
orthophotography
oxyhydrogen
plumb bob
projection
quadrant
quicklime
spheroid
sighting instrument
theodolite
topographic map
topological map
Transverse Mercator
triangulation
(true) origin.