**AN ON-LINE GEODATA COLLECTION ACTIVITY TO BE HOSTED BY THE SOCIETY OF AFRICAN EARTH SCIENTISTS 0N 20**

^{TH}MARCH 2013 AS PART OF DAY OF EARTH SCIENCES IN AFRICA

The Society of African Earth Scientists
(SAES) will on the 20

^{th}March be participating in “Day of Earth Sciences in Africa “ as part of the Association of African Women in the Geosciences-led, “Day of Earth Sciences in Africa and the Middle East”. The Society, as a partner in the event, will be hosting an on-line geodata collection activity involving the measurement of the length of Africa’s coastline and the area of Africa’s landmass. All participants are invited to post their results on the SAES facebook page on the 20^{th}March or email their results to saescientists@hotmail.co.uk.

*Activity Summary*
To measure the area and coastline of
Africa’s landmass in square km and km respectively, using Google earth software.
Google earth software is downloadable free from the Internet.

*Outcome*
The SAES
hosted activity will

- encourage African scientists
and interested public to become familiar
with a potential earth science tool that is freely available

- enable appreciation of the true
size of the African continent, until recently unknown.

*Background*
The advent of
satellite technology and the Internet has liberated the African scientist from
having to accept, based on faith,
statistics of his/her own continent as measured by others. Until 2010, the size of the African Continent
appearing on most maps, made Africa 14 times smaller than its actual size! This
is because the Mercator Projection (
published by Gerardus Mercator in 1569) on which most maps are based till
this day, failed to correctly allow for the curvature of the Earth, thereby
distorting true values of African continental size on earlier maps. In 2010 the
computer graphics expert Kai Krause published a map of Africa which showed
other countries crammed into its outline.
It gave, at a glance, an appreciation of the true size of Africa, which
is able to encompass the United States, Mexico, Japan, China, Iberia peninsula,
India and western Europe including the UK
.

*Detailed Description of the Activity*
The Society of African Earth Scientists’
Earth Science Day activity invites us to measure two very important quantities:
the area and the coastline of the African continental landmass. The tool we
recommend for this exercise is Google earth. Google earth can be freely
downloaded from the internet, and measures distances on the surface of the
earth. Google earth (assuming we do not have Google earth Pro, which measures
areas directly) allows us to measure Africa’s coastline very easily; whilst we
need to work a bit harder to get an estimate of area by measuring only
distances.

Ideally, we could get a very good estimate of
Africa’s area by supposing that the shape of Africa is made up of an array of very
thin rectangles of equal width, d, (see fig. 1 above). This fine estimate can in theory be made
easily as it is easy to measure the lengths of each rectangle drawn within the
shape of the continent. Since each rectangle is of width d, and the area of a
rectangle is its length multiplied by its width, the area of the continent is thus
given by the sum of the lengths of all the constituent rectangles multiplied by
their common width, d. The smaller the value of d, the more accurate is the
estimate. Theoretically, if the width d is infinitely small, the estimate is
exact[1].

In
practice, it is very time consuming to measure area by making the width, d, so
fine as to require the use of many rectangles to fill up the shape of Africa
approximately. Also it is not easy to ensure the measurement of lengths which
are perfectly equidistant as in the case of the idealised rectangles.

A
quicker method which still gives us an acceptable accuracy within 5% is to
approximate the shape of the continent as closely as possible using an
arrangement of rectangles and triangles to occupy the space inside the outline
as closely as we see fit. We then determine the areas of these rectangles and
triangles in the usual way

*(i.e., the area of a triangle is ½ x base x height; whilst that of a rectangle is width x length)*, and we sum all the areas to obtain the estimate of the area of Africa’s landmass. We note that making use of respectively thinner and smaller (and therefore more numerous) rectangles and triangles will improve the accuracy of our estimate; but will be more time consuming. We must balance the benefits of one against the other, accuracy against labour saving, depending on the level of accuracy we need to achieve.
Having
appreciated the method by which we approach our task we proceed to the
measurement of distances in Google earth.
In the tool bar at the top of the google earth screen, there is a symbol
of a ruler. There is also the symbol of dots connected by a line. These icons are a clue as to how we are to
use Google earth to measure distances.

i)

*Measurement of the African coastline using Google earth*
To
measure the distance around the outline of the African continent, we click on
the icon showing dots connected by a line. Hovering over the icon reveals
the label “add path “over the icon. A window will appear with various tabs,
including one labelled “measurements”. Click on the “measurements” tab and a
“new path” window will appear with a space
for the display of length. At this point you may choose from a drop down list
in this space to display these distances in units of your choice. For this
exercise, we choose kilometres (km).

We then use the mouse cursor to trace a path
around the outline of Africa. When we have completely traced the coastline as
closely as possible, the window gives us the distance (length) in kilometres.
Your answer should be close to 26,000 km.

*ii) Measurement of the Area of Africa’s Landmass*
As
described above, we begin by sketching an arrangement of rectangles and
triangles to fit inside the outline of the African continent as closely as
possible (see fig 2 ). We then use this sketch as a guide for the distances we
are going to measure using Google earth’s ruler facility. We proceed to measure
the required distances. We will not trace as we did in the last activity, but
instead at each step measure the distance between two points.

We begin with the first rectangle side (or
triangle side, depending on our arrangement). We click on the ruler icon in the
top tool bar. A window appears with two tabs (“line and “path”) and we choose
the tab labelled “line”. This
time we click on the upper point of a vertical rectangle side (for instance) on
Africa’s north coast, the length of which we intend to measure. Then we click on
the lower point at the bottom end of this vertical side. A line appears joining
the two points and the window shows the length of the line in kilometres. We write
down the length of this line on our outline sketch of Africa (as shown in Fig.
2). We continue in this fashion until
all the lengths of rectangle and triangle sides in Fig. 2 have been measured.
We then proceed to determine the areas of the rectangles and triangle shapes
composing the shape of Africa and sum them up to arrive at our total continental land area as described previously.

[1] This
is the principle behind estimation of geometric areas and other
quantities in mathematics by means of the operation known as “integration”

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