With suitable examples, discuss the importance of accurate

geolocation

Introduction/ Historical Background

The science of geolocation is concerned with the methods and

concepts used to locate objects and features on the Earths

surface, and in nearby space with respect to a terrestrial

coordinate system. Geolocation and its related science of

geodesy have evolved over historical time, from the verification

of the spherical Earth model by Erastothenes in 1000 B.C.

(Logsdon, 1995, p194), the introduction by Gerardus Mercator

of his conformal map in 1569, that allowed for the construction

of charts suitable for ‘rhumb line’ navigation (Kellaway, 1957,

pp38-39), to modern insights into the shape of the Geoid, and

navigation using the ‘Global Positioning System’ (GPS)

(Logsdon, 1995).

In the fourteenth century, and for several hundred years

afterwards, geolocation at sea was hampered by the problem that

only latitude could be determined quantitatively, by means of an

astrolabe or sextant; longitude being estimated by little more

than guess-work. This resulted in a number of maritime

disasters, culminating in the wrecking of the British fleet off the

Scilly Isles in 1707, under the command of Sir Cloudsley

Shovel. In response to this problem the British Board of

Longitude was established, and in 1714 offered a cash prize to

any person who could determine the east-west position of a ship

to within 30 nautical miles (Logsdon, 1995, p158). The

‘longitude problem’ was eventually solved in the 1760’s with the

invention of an accurate marine chronometer (accurate to one

second of loss per day, corresponding to a positional error of ten

nautical miles after six weeks at sea) to provide timed sextant

sightings of celestial objects; since the time of the equivalent

celestial sighting (a rising for instance) would be known at

Greenwich and the earth rotates at 15 degrees per hour, the

longitude could be determined (Logsdon, 1995, p160-161).

During the nineteenth century celestial navigation/survey

methods were also used on land in continental reconnaissance

surveys.

In 1687 Isaac Newton proposed that the shape of the earth is

controlled by hydrostatic considerations such that the

combination of gravitational and centripetal effects would result

in an oblate spheroid (ellipsoid) with a flattening of one in 230,

the modern value is one in 300 (Logsdon, 1995, p194-195). This

development is significant from a modern geolocation

perspective since ellipsoid models provide the underlying basis

of many mapping agencies coordinate systems. Since no

ellipsoid can perfectly match the true shape of the earth various

ellipsoids have been defined to model the whole earth (global

ellipsoids), or to model various geographic regions (regional

ellipsoids) (Ordnance Survey, 1999, p6).

Ellipsoid Datums

In order to define the position of a point on the earths surface

using an ellipsoid coordinate system, the latitude and longitude

on the stated ellipsoid must be given in addition to the distance

from the point to the ellipsoid surface along the normal vector to

the ellipsoid surface. This is the so-called ‘ellipsoid height’

(Ordnance Survey, 1999, p9). It is essential to know which

ellipsoid is being used, because if several points with the same

stated latitude/longitude, but defined in different but valid

ellipsoid coordinate systems are compared, these points can be

up to 500 metres apart on the ground (Ordnance Survey, 1999,

p4).

Most nations have a ‘geodetic network’ that defines the

relationship in space of a particular ellipsoid to fixed physical

points on the topography of that country. Before 1950 such

primary national geodetic networks were usually created by

visual triangulation between triangulation points (trig points),

forming a triangulated or ‘geodetic’ mesh over the territory of

interest. A few short sections of the network would be measured

along the ground to scale it, and these sections world have their

orientations measured astronomically to orientate the entire

network (Jones, 1999, p16). Root-mean-square (RMS) residual

techniques could then be employed to determine a best-fit

ellipsoid to that geodetic network, and to formally define the

spatial relationship between the network (real physical points in

space) and the ellipsoid (a mathematical object). Such a formal

definition is referred to as a Datum definition (Jones, 1999, p16).

Modern techniques such as GPS (using signals from Global

Positioning Satellites) are capable of sub-centimetric accuracy

when used differentially (DGPS) with survey grade equipment,

and are being used to replace visual triangulation in primary

geodetic networks (Jones, 1999, p16), (Ordnance Survey, 2000,

p1).

Comparison of traditional and DGPS derived primary geodetic

calculations for the UK indicate that on large scale mapping

(1:2500), global horizontal errors of up to 9 metres may have

been present in some areas, although local errors between

adjacent surveyed objects were within specification (Ordnance

Survey, 2000, p4). Such issues of absolute versus relative

positional error are very important if old and new map

coverages, of the same scale, are being overlaid in a

Geographical Information system (GIS), otherwise false

topological relationships may result (Ordnance Survey, 2000,

p1), (Mackaness, pers com), (Burrough, et al, 2000, p224).

If geolocation data needs to be translated from one datum

(ellipsoid coordinate system) to another, a seven parameter

mathematical transformation needs to be carried out (Jones,

1999, p16). Since the parameters have been determined by

observation this will introduce more positional error into the

data. In addition most datum’s change in position with respect to

each other over time due to tectonic movements within the

earths crust (Ordnance Survey, 1999, p5), hence corresponding

transformation parameters will also change.

Illustrative Examples

Geolocation data of differing positional accuracies can be

combined for interpretation purposes (Ganas, et al, 2001), but it

essential that the accuracies of the various datasets be taken into

account by the interpreter in order to avoid incorrect conclusions

and false topological inferences (Mackaness, pers com),

(Burrough, et al, 2000, p224).

Ganas et al (2001) carried out a combined analysis of Landsat

TM images, field observations and aftershock distribution

patterns in order to determine on which of two candidate fault

planes the 7th September 1999 Athens 5.9 Ms earthquake

occurred. The candidate fault planes were separated by only 3km

in the mesoseismic area. Seismological locations for the initial

epicentre and aftershocks were accurate to only +/- 1km in x,y

and z directions, insufficient to determine the active fault. A

Landsat TM image was taken a few days after the earthquake,

with a spatial resolution of 30 metres. This image was geo-

referenced to the EGSA datum by a second order polynomial

transformation, using ground control points (gcps) from 1:50000

scale maps; gcp rms-error being less than one pixel (30m). Field

mapping of ground surface breaks was done using a hand held

GPS with an accuracy of +/- 60 metres x,y . Theses GPS

locations were translated from WGS84 datum to EGSA datum

using a standard datum transformation. A 20 metre gridded

DEM was produced from contour elevations on the 1:50000

maps (20m contour interval), in order to created a shaded relief

image to use as a backdrop for other data layers. The geo-

referenced Landsat image was draped over the DEM to create

3D perspective views on which other data could be plotted.

These combined geo-referenced datasets were interpreted,

resulting in a positive identification of the active seismic fault.

Thus, the relatively low accuracy seismically derived

geolocations were supplemented with the higher accuracy

geolocations of other pertinent data sets in order to determine the

active fault.

Digital elevation models and topographic maps usually contain

‘orthometric elevations’ rather than ellipsoid heights.

Orthometric heights correspond to the perpendicular height

above the gravitational equipotential surface, or Geoid. The

Geoid is the global surface, which corresponds to the ‘common

sense’ notion of the ‘level-surface’ in that ‘plumb-line’ verticals

hang at right angles to it (Ordnance Survey, 1999, p7). Without

the effect of wind, currents and tide the Geoid would be close to

global sea level.

Hofton et al (2000) demonstrated that airborne laser altimetry

was able to determine orthometric heights over the Long Valley

caldera in California to between 3 and 10 cm. The aircraft used

was equipped with dual frequency DGPS receivers in order to

calculate trajectory relative to a fixed ground station. Aircraft

attitude was determined from an inertial navigation system

(INS). Positional data from these navigation systems were

combined with laser altimetry readings in order to obtain the

location and mean elevation of each measurement ‘footprint’ of

the system. The resulting ellipsoidal heights in the WGS84

datum were converted to orthometric heights using the

GEOID96 model. Over-flights of a lake in the test area indicated

that orthometric heights given by the system matched the

expected local geoid profile of the water surface to within 3cm.

Hofton et al (2000) state that such high accuracy geodetic

monitoring techniques represent a valuable tool for detecting

topographic uplift in resurgent calderas.

Conclusion

From the specific examples given it can be shown that accurate

geolocation is essential for safe navigation, the successful

solution of geoscientific problems, and the early identification of

natural geological hazards. In addition, accurate geolocation is

needed for any discipline that needs to known the absolute or

relative position of any object with respect to the earth. By

‘accurate’ we mean of a sufficient and stated accuracy for the

purpose required. Thus all geolocations should include a

statement of their precision and accuracy, as part of their meta-

data (Heywood, et al, 1999, p178).

References:

Burrough, P. A., and McDonnell, R. A., 2000, Principles of

Geographical Information Systems. Oxford University Press,

p333.

Ganas, A., Papadopoulos, G., and Pavlides, S. B., The 7

September 1999 Athens 5.9 Ms earthquake: remote sensing and

digital elevation model inputs towards identifying the seismic

fault. Int. J. Remote Sensing,2001, vol. 22, no. 1, pp191-196.

Heywood, I., Cornelius, S., and Carver, S., 1999, An

Introduction to Geographical Information Systems. Longman,

Harlow, p279.

Hofton, M. A., Blair, J. B., Minster, J. B., Ridgeway, J. R.,

Williams, N. P., Bufton, J. L., and Rabine D. L., An airborne

scanning laser altimetry survey of Long Valley, California. Int.

J. Remote Sensing,2000, vol. 21, no. 12, pp2413-2437.

Jones, A., 1999, Where in the World are We? (Version 1.7).

Department for Environment, Heritage and Aboriginal Affairs,

Adelaide, South Australia, p27.

Kellaway, G. P., 1957, Map Projections, Methuen & Co Ltd,

London, p

Logsdon, T., 1995, Understanding the Navstar GPS, GIS, and

IVHS (Second Edition).Van Nostrand Reinhold, p329.

Ordnance Survey, 2000, Information paper 1/2000, Coordinate

positioning, Ordnance Survey policy and strategy. Ordnance

Survey, p7.

Ordnance Survey, 1999, A guide to coordinate systems in Great

Britain. Ordnance Survey, p44.

PG STUDENT MATRIC. NO. 0197746

M.Sc. GIS, Geomatics Essay 1500

words