Hydrographic Data and GIS
81
Hydrographic offices are used to receiving data from many different sources
and have well established procedures for assessing its reliability. All data used in the
trial with the OS was assessed as part of the standard screening process used to ensure
that only valid data is included in the HO's navigational products.
Most users will wish to know something about the accuracy of the data they
buy. Many users will want the data to meet certain minimum accuracy standards. For
some applications meaningful results can only be achieved with very accurate data. For
example, if hydrographic surveys are used to monitor the volumes of spoil removed
from a navigational channel, or the quantity of aggregates dredged from an offshore
bank, depths have to be measured to within +/- 0.1 metre and fixed horizontally to
within +/- 1 to 3 metres. Most applications, however, have less stringent accuracy
requirements. Less accurate data may be used for most modelling and mapping
applications.
Most users require data that is up-to-date. If the data is not up-to-date, users
generally require an assurance that it is still extant or, failing this, an indication of how
much change may have taken place since the data was collected. Some applications
require historic as well as current data, for example, to monitor seasonal or annual
changes in sea water quality, or long term changes to the position of a coastline or
offshore features such as sandwaves. Users also require data to be complete. As far
as possible there should be no gaps in the area covered by the data. Finally, the data
should be consistent. Ideally all of the data should be equally reliable and of the same
scale, accuracy and age.
Data Compatibility
Data has to be compatible with the GIS in which it is to be used. This is not a major
issue for serious producers of data for GIS. Having created the basic data it is a
relatively easy task to develop or acquire the software needed to output the data in a
range of different formats, such as NTF (National Transfer Format - British Standard
7567), DXF (Data eXchange Format) and a variety of other proprietary formats.
Creating the basic data can be far more problematic, especially if the dataset has to be
created by combining data from a number of different sources. This was the case
during the second stage of the HO/OS coastal zone mapping project. During this
stage of the project, hydrographic data from two different sources was combined with
topographic data from another six sources.
The aim of the project was to create a digital coastal zone map made of
intelligent vector data. None of the data used in the trial was already in this form.
Intelligent vector data had to be created either by reprocessing existing digital data or
by digitising the data directly from repromat. With the exception of some additional
drying heights, all of the hydrographic data had been digitised as part of the normal
production process for new paper charts. Although feature codes were included in the
data, these referred to the linestyles that were to be used for reproducing the chart. In
this form the data could only be used within a GIS to create a digital image of the
original paper chart. To enable the data to be displayed selectively, queried and used
81
Hydrographic offices are used to receiving data from many different sources
and have well established procedures for assessing its reliability. All data used in the
trial with the OS was assessed as part of the standard screening process used to ensure
that only valid data is included in the HO's navigational products.
Most users will wish to know something about the accuracy of the data they
buy. Many users will want the data to meet certain minimum accuracy standards. For
some applications meaningful results can only be achieved with very accurate data. For
example, if hydrographic surveys are used to monitor the volumes of spoil removed
from a navigational channel, or the quantity of aggregates dredged from an offshore
bank, depths have to be measured to within +/- 0.1 metre and fixed horizontally to
within +/- 1 to 3 metres. Most applications, however, have less stringent accuracy
requirements. Less accurate data may be used for most modelling and mapping
applications.
Most users require data that is up-to-date. If the data is not up-to-date, users
generally require an assurance that it is still extant or, failing this, an indication of how
much change may have taken place since the data was collected. Some applications
require historic as well as current data, for example, to monitor seasonal or annual
changes in sea water quality, or long term changes to the position of a coastline or
offshore features such as sandwaves. Users also require data to be complete. As far
as possible there should be no gaps in the area covered by the data. Finally, the data
should be consistent. Ideally all of the data should be equally reliable and of the same
scale, accuracy and age.
Data Compatibility
Data has to be compatible with the GIS in which it is to be used. This is not a major
issue for serious producers of data for GIS. Having created the basic data it is a
relatively easy task to develop or acquire the software needed to output the data in a
range of different formats, such as NTF (National Transfer Format - British Standard
7567), DXF (Data eXchange Format) and a variety of other proprietary formats.
Creating the basic data can be far more problematic, especially if the dataset has to be
created by combining data from a number of different sources. This was the case
during the second stage of the HO/OS coastal zone mapping project. During this
stage of the project, hydrographic data from two different sources was combined with
topographic data from another six sources.
The aim of the project was to create a digital coastal zone map made of
intelligent vector data. None of the data used in the trial was already in this form.
Intelligent vector data had to be created either by reprocessing existing digital data or
by digitising the data directly from repromat. With the exception of some additional
drying heights, all of the hydrographic data had been digitised as part of the normal
production process for new paper charts. Although feature codes were included in the
data, these referred to the linestyles that were to be used for reproducing the chart. In
this form the data could only be used within a GIS to create a digital image of the
original paper chart. To enable the data to be displayed selectively, queried and used