text
stringlengths 0
34.3k
|
|---|
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
|
Downloaded from https://onlinelibrary.wiley.com/doi/ by Calis-China University Of, Wiley Online Library on [07/09/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
|
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.
|
End of preview. Expand
in Data Studio
README.md exists but content is empty.
- Downloads last month
- 1