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SGML, HTML and XML -
What are they???
The difference between HTML and XML is the intent of the two. XML is
something close to HTML.
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SGML is the father of the markup languages. It was created
by Dr. Charles Goldfarb in the mid 80's as a way to standardize the
identification and collection of data. Many people will tell you that
SGML is not a language itself but a way to create languages. Those
people want to scare you - ignore them. Its not that complex.
Just think of it as a language. You also don't have to learn it to
understand what follows.
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HTML is the markup system used in the world wide web. This
page is marked up in HTML and If you would like to see an example just press
View | Source on your Internet Explorer and you will see the code to create
what you are reading now. HTML is used to define the Style
of the data. In HTML you define the style of the text with markup
like <FONT>, <I> for italics, <B> for bold, and <U>
for underline tags. You also can define the Structure
of the document with tags like <Hn> for heading and <P>
for paragraph. HTML also allows you to describe some Content
of the data with tags like <TITLE> and <CODE>.
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XML is used to define the Content
of the data by allowing itself to be extended to fit the
circumstances. This is exactly why it is valuable to SETI.
The ability to define the content is exactly what is needed for SETI data
collection. We need a way to save data and later review it or share it
"without regard to it's look". We need a way to compare
data from Argus stations collected at different times and on different types of
equipment. Look is unimportant when the data is generated and analyzed
strictly by computer.
XML is a subset of SGML. When a new language is defined, in
either SGML or XML, its definition is recorded in a Document Type
Definition (DTD). For example HTML, the standard language used to create
the WWW page you are viewing, is described in a DTD. The HTML DTD can be
viewed at the World Wide Web consortium home
page. When revisions to HTML are made they are made to the
DTD.
XML has most of the features of SGML with some of the more cumbersome
structures removed. This makes it perfect for our needs. One of the
differences is that XML languages, like SML, are usually defined in a schema
rather than a DTD. The schema itself is written in XML so there is only
one language to learn. The SML schema is available to download.
It turns out that a astronomical markup language has been defined and
committed to a DTD. This language is the Astronomical
Markup Language (AML). AML has been examined and forms the core of our
SETI DTD.
Advantages of Starting With
XML in the creation of SML
| New |
This is the cutting edge technology. I want to be part
of that. I do this for a hobby not because I have to so why not
learn something brand new. |
| Object Oriented |
The ability to nest XML elements means that a data set can
be built as an object. One observation object could contain
frequency objects, amplitude objects, time objects etc. |
| Existing DTD for a model |
AML and AIML exist and can be used a starting point for the
SETI Markup Language. |
| Compatible with current browsers |
The most recent versions of Netscape (version 4.7) and
Internet Explorer (version 5.0 and above) will display an XML (and
therefore SML) document directly in limited form. |
| XML can be embedded in a standard web document |
An "island of XML" can be put into a HTML document
which can be accessed in various ways by the users browser. This way
SML can be viewed by the casual visitor to a web site with no specialized
tools. |
| Netscape and Microsoft are committed to XML |
Both companies latest versions incorporate at least some of
the features of XML. Both companies are expected to make XML the
bases for future products. SML will benefit from this joint work
because the tools will be maintained for us and the specification will be
professionally managed. |
| Resulting Data is Searchable |
This is the key to XML and thereby SML. Each data
point will have an element name that can be used to place the data in a
data base.
If designed correctly SML data could be used for many
purposes. For example the system noise temperature could be computed
directly from the data. Two searches from different Argus stations
could be combined and normalized for a single, new, view.
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One of the items defined in SML is the Argus stations location. The
data below defines the method of location identification used by Argus.
From Art Lange W6RXQ - Argus Station CM87XI
Here's
an example of how the size of the grid (micro grid) decreases as the
Maidenhead precision increases:
CM87XI42LF16
the grid sizes for different precision are:
CM = 10 X 20 degrees
CM87 = 1 X 2 degrees = approx. 69 X138 miles at the equator (1
degree = 60 nautical mile/degree at the equator)
CM87XI = 2.5 X 5 minutes = approx. 2.87 X 5.75 miles at the
equator (extra letters divide by 24)
CM87XI42 = 0.25 X 0.5 minutes = approx. 0.287 X 0.575 miles at
the equator (extra numbers divide by 10)
CM87XI42LF = 0.01 X 0.02 minutes = approx. 0.0114 X 0.02296 miles
= 60.6 X 121.2 feet at the equator (extra letters divide by 25)
CM87XI42LF16 = represents a grid square approximately 6 X
12 feet at the equator. (extra numbers divide by 10).
For
those interested in correcting the size of the grid square for
different latitudes, use the cosine of the latitude to shorten the
longitude (east-west) readings. For example at 37 deg north, 1 minute
difference in longitude = cos(37) X 1 nautical mile = 0.798 nm.
Note: It is important to use the same divisors for the letters (divide
by 25 for the 3 rd and subsequent letter pairs.) (In Maidenhead the
first letter pair divides by 18 and the second divides by 24. The
extensions divide by 25, which keeps decimal minutes rational).
The beauty of the extended Maidenhead designators is the efficiency of
the representation. To represent the same precision in decimal degrees
requires lots more characters. For example 3 feet = approx. 0.000001
degrees. Thus to represent my 12 character full precision grid
(CM87XI42LF16) to 6 feet precision, it takes 21 characters in decimal
degrees (37.383345 N 122.033456
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