Datum Query Help
A datum is a mathematical model of the Earth used to calculate coordinates
on maps, charts, and survey systems. LAS currently supports 221 datums. Each
datum is identified by a LAS datum code and a datum name. Datums are defined by
a spheroid and x,y, and z shifts. A spheroid is a mathematical surface used to
approximate the Earth's surface and is defined by a semimajor axis and a
semiminor axis. Datums usually apply to certain countries or regions.
The datum query program allows you to query on
specific fields and displays all datums that match the query. The user can also
choose which datum information to have displayed.
Help Topics:
The following fields and options specify search criteria. The lack of a
criterion indicates that all values for the corresponding field are valid for
the search. If the "Query Selection" option is selected, at least one field
must be specified. If the "Display All Datums" option is selected, the search
fields will be ignored and all datums will be displayed when the query is
submitted.
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- Search only for datums whose names contain the input text. The user may
also use the wildcard character, *.
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- Search only for the datum with the specified datum code. You can enter any
single valid datum code in the range of 100 to 320. Any invalid numeric entry
or an entry outside this range will result in an "Invalid datum code" error.
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- Search only for datums defined by the selected spheroid name. The user can
select a specific spheroid name from the given list. The user may also use the
datum query tool to view information for a spheroid not associated with any
datum. To do this, leave all other fields empty and specify the spheroid here.
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- Search only for datums defined by the specified spheroid code. The user can
enter any single valid LAS spheroid code in the range of 0 to 31. Any invalid
numeric entry or an entry outside this range will result in an "Invalid
spheroid code" error. As with the spheroid name, the user may also use the
datum query tool to view information for a spheroid not associated with any
datum by its spheroid code. To do this, leave all other fields empty and
specify the code here.
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- Search only for datums with the selected category. The user can select a
specific category from the given list.
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- Use the "Query Selection" option to query on the input search criteria. Use
the "Display All Datums" option to display all datums when the query is
submitted. If the "Display All Datums" option is selected, all other input
search criteria is ignored.
Use the following options to select the format in which the datum
information is displayed.
- Datum Name/Code
- Display the datum name and code in the following format:
Datum : (Datum Code) Datum Name
- Spheroid Information
- Display the spheroid information associated with the datum. Spheroid
information displayed includes the spheroid code, name, semimajor axis and
semiminor axis. This option must be selected when querying for a spheroid not
associated with a datum or no information will be displayed.
- Region
- Display the region with which the datum is associated.
- Category
- Display the datum's category.
- Datum Shifts
- Display the X shift, Y shift and Z shift.
- Display All
- Display all information avaliable about the datum. If this option is
selected, the other output format options are ignored.
Once the query fields have been specified and the output format has been
selected, the query can be submitted by clicking the "Submit" button. Note that
if the "Query Selection" option is specified, the user must specify at least
one of the query selection fields. Failure to do so will result in a "No input
information to query on" error.