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CHAPTER 1 |
INTRODUCTION |
1.1 INTRODUCTION |
We currently live in what is often termed the information age. Aided by new |
and emerging technologies, data are being collected at unprecedented rates in all |
walks of life. For example, in the field of surveying, total station instruments, |
global navigation satellite systems (GNSS), digital metric cameras, laser-scanning |
systems, and satellite imaging systems are only some of the new instruments that |
are now available for rapid generation of vast quantities of observed data. |
Geographic information systems (GIS) have evolved concurrently with the |
development of these new data acquisition instruments. GIS are now used exten sively for management, planning, and design. They are being applied worldwide |
at all levels of government, in business and industry, by public utilities, and in |
private engineering and surveying offices. Implementation of a GIS depends on |
large quantities of data from a variety of sources, many of them consisting of |
observations made with the new instruments, such as those noted above. |
However, before data can be utilized, whether for surveying and mapping |
projects, for engineering design, or for use in a geographic information sys tem, they must be processed. One of the most important aspects of this is to |
account for the fact that no measurements are exact; that is, they always contain |
errors. |
The steps involved in accounting for the existence of errors in observations |
consist of (1) performing statistical analyses of the observations to assess the |
magnitudes of their errors and to study their distributions to determine whether or |
not they are within acceptable tolerances; and if the observations are acceptable, |
(2) adjusting them so that they conform to exact geometric conditions or other |
Adjustment Computations: Spatial Data Analysis, Fifth Edition, Charles D. Ghilani |
2010 by John Wiley & Sons, Inc. Published by John Wiley & Sons, Inc. |
1 |
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2 INTRODUCTION |
required constraints. Procedures for performing these two steps in processing |
measured data are principal subjects of this book. |
1.2 DIRECT AND INDIRECT MEASUREMENTS |
Measurements are defined as observations made to determine unknown quantities. |
They may be classified as either direct or indirect. Direct measurements are |
made by applying an instrument directly to the unknown quantity and observing |
its value, usually by reading it directly from graduated scales on the device. |
Determining the distance between two points by making a direct measurement |
using a graduated tape, or measuring an angle by making a direct observation |
from the graduated circle of a total station instrument, are examples of direct |
measurements. |
Indirect measurements are obtained when it is not possible or practical to |
make direct measurements. In such cases the quantity desired is determined from |
its mathematical relationship to direct measurements. For example, surveyors |
may observe angles and lengths of lines between points directly and use these |
observations to compute station coordinates. From these coordinate values, other |
distances and angles that were not observed directly may be derived indirectly |
by computation. During this procedure, the errors that were present in the origi nal direct observations are propagated (distributed) by the computational process |
into the indirect values. Thus, the indirect measurements (computed station coor dinates, distances, and angles) contain errors that are functions of the original |
errors. This distribution of errors is known as error propagation. The analysis of |
how errors propagate is also a principal topic of this book. |
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