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#1
haris179

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hi there!

does anyone have an idea about GML? cuz i have never used it before and now i am doing R & D on this. if anyone can help me will highly be appreciated.

cheers.

#2
James Hines

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Here just do a look-up on the internet. But first you can start here:

http://en.wikipedia....Markup_Language

"There is much beauty that we fail to see through our own eyes teeming with life forms that give us that perception of our reality.  Leaves on the trees blowing gently in the wind, or scarily, the waves pounding through high surf, or lightly on a warm summer’s day; that opportunity to sit or swim in the water on a white beach.   That comfort to shout, “The universal conscious do you hear me?  I am alive, guide me dear logos towards the path of rightnesses.”  Earned what has been kept, no longer to be absorbed into a life filled with cold damn winds and  that stubborn fog clouding  my vision with nothing but darkness."


#3
Esther Mandeno

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hi there!

does anyone have an idea about GML? cuz i have never used it before and now i am doing R & D on this. if anyone can help me will highly be appreciated.

cheers.


What do you need to know? It's basically like XML, a text based mark up file. I've written my own GML files, fairly simple and there are lots of automated programs out there. If you use ArcMap, there's a script out there that will convert your shapefiles to KML files (which is Google Earth's version of GML). The link Hasdrubal gave you should get you started in the right direction though. Good luck!
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#4
Hans van der Maarel

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I'm quite tempted to write out a big essay explaining exactly what I think is wrong about GML. To make a long story short, it's workable if it's an agreed upon standard that is supported by the software you use. If not, and you have to deal with an unknown GML file, you're in for a lot of pain.

Aside from that the files are terribly bloated. It used to be said that this would be no problem since hard drives were cheap. Well, they are, but sending GML files online still takes a lot of time, so does parsing them.
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#5
frax

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compressed they are not that bad in file size. Regarding Esther's comment, kml is not equivalent to GML - KML also includes presentation specifications, in addition to the raw data.
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#6
Carl Reed

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Step back a minute. First, what is the problem you are trying to solve? Remember, GML is an XML grammar for encoding geographic content. If you wish to encode geographic content in a human readable, self describing, interoperable encoding then GML may be the way to go. If you are working at visualizing relatively geographic data in an earth browzer application, then KML may be the way to go. All depends on your requirements and the problem you are trying to solve.

Now, as to the criticisms of GML:

1. Using GML works best when you have a content model, such as those developed and used in the geological, aviation, and meteorological communities. Good examples of content models and related GML encodings of those models can be found on the FGDC web site: http://www.fgdc.gov/...el-description/ . If you just start "coding" in GML, then you definitely will have problems.

2. GML bloat. Guess what. Andrew Turner did an interesting analysis of file sizes using a variety of encodings for a land and water dataset:

Shapefile 5.4 MB 3.6 MB
GeoRSS GML 3.3 MB 1.1 MB
KML 7.3 MB 2.4 MB
Spatialite 5.4 MB 3.6 MB
JSON 7.9 MB 2.3 MB

First column is uncompressed and the second column is zipped. GeoRSS GML was the smallest file!

3. Ability of applications to use GML. At this time, the majority of GIS packages support the ability to create and ingest GML. You only need to make sure that the version numbers for GML are the same. Otherwise, you might have problems. GML 3.1.1 is the most widely used version although version 3.2.1 is being widely implemented now.

If you want more resources regarding GML applications, check out http://www.ogcnetwork.net/gml

Hope this helps!

compressed they are not that bad in file size. Regarding Esther's comment, kml is not equivalent to GML - KML also includes presentation specifications, in addition to the raw data.



#7
Hans van der Maarel

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2. GML bloat. Guess what. Andrew Turner did an interesting analysis of file sizes using a variety of encodings for a land and water dataset:

Shapefile 5.4 MB 3.6 MB
GeoRSS GML 3.3 MB 1.1 MB
KML 7.3 MB 2.4 MB
Spatialite 5.4 MB 3.6 MB
JSON 7.9 MB 2.3 MB

First column is uncompressed and the second column is zipped. GeoRSS GML was the smallest file!


All the GML datasets I've come across have been, uncompressed, on average been 10 to 100 times larger than their shapefile equivalents, with processing times being exponentially longer. This makes it, imho, absolutely not suitable for large amounts of data.
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#8
frax

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One should see GML as a transfer format, and not as a working format (e.g. for analysis and processing) or storage format (possible for archiving).

It being XML, advantages are partly outside the realm of GIS - it makes it very easy to query it (using e.g. XPath, or by just parsing it - I am referring to non-spatial queries) to parse it and to transform it - to e.g. SVG, KML, RSS or even HTML (also as data islands).

For effective processing, I don't the size is the key, it is more the lack of indexes, both spatial and for attributes.
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#9
pghardy

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This makes it, imho, absolutely not suitable for large amounts of data.

I think that's a bit harsh :) .GML is a flexible interchange format, not a live database format. It is verbose, but compresses well. In terms of 'large amounts of data', the Ordnance Survey successfully uses GML to distribute the MasterMap data of Great Britain - nearly half a billion features, which is large by anyone's standards!
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#10
Hans van der Maarel

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I think that's a bit harsh :) .GML is a flexible interchange format, not a live database format. It is verbose, but compresses well. In terms of 'large amounts of data', the Ordnance Survey successfully uses GML to distribute the MasterMap data of Great Britain - nearly half a billion features, which is large by anyone's standards!


The Dutch Topographic Service has opted for GML too and their production files take hours of processing, rather than minutes for comparable shapefiles. If I compare it to their old production format (E00) the difference is less dramatic though.

So tell me this:
What is the big advantage of using GML over using one of the old proprietary formats? What can we do now that we couldn't do when we were still using Shape, or E00, or DXF, or Mid/Mif, or... ?
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#11
frax

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What is the big advantage of using GML over using one of the old proprietary formats? What can we do now that we couldn't do when we were still using Shape, or E00, or DXF, or Mid/Mif, or... ?


Human readable, easily extendable, supports other/any encodings, no restrictions in attributes etc, easily accessible by a variety of tools (including non-GIS - see my reply above), streamable (as in WFS), easy to transform (SVG, KML, HTML etc). Without further insight into GML specifications, I think it might also better support complicated geometries.

The only one that you list above that I would say is a real transfer format is E00, which is ancient...
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#12
Hans van der Maarel

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The only one that you list above that I would say is a real transfer format is E00, which is ancient...


I don't actually make a distinction between transfer formats and other formats. I don't think many other people do. Data is stored in a format. My job (when I'm doing FME stuff) is to put it in another format. If I look at the real world scenarios where GML (or XML for that matter) have been involved, I keep seeing problems that really should not be there. Most of that probably isn't totally GML's fault, but rather a shoddy implementation.

Trust me, I'm not on some sort of crusade against GML here. But the implementations that I have come across didn't really work out for me and in the cases where I could choose between GML and an ancient proprietary closed format, I invariably ended up choosing the way of the dinosaur, after testing both.
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