Download gml file

GML is an abbreviation for "Geography Markup Language." A spectacularly inefficient format, GML is based on XML and is being touted as a proposed standard in some circles. Unfortunately, GML is something of a non-standard since every implementation of GML to date for saving GIS data has been incompatible with other implementations. Besides the intrinsic incompatibility designed into GML, the main problem with GML is the extreme inefficiency of the format: GML will frequently require files that are over megabytes in size to store GIS data that other formats can save in only five megabytes.

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GML is currently used by the UK's Ordnance Survey (OS) for publishing MasterMap data products, the only concrete form of GML that is currently used to publish a significant amount of data. The Manifold GML importer is therefore an importer for Ordnance Survey GML format, specifically, for OS MasterMap data products. www.cronistalascolonias.com.ar expects to work closely with the OS to assure increasing compatibility with OS usage of GML.

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GML files may have a .gml extension or, as is the case with some Ordnance Survey (OS) data files with no extension. GML files can occur in both independent and topological polygon forms. Manifold will automatically import either type and will apply any updates specified. Manifold's GML importer supports object versions and will import any metadata as a Manifold comments component.

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One interesting aspect of GML format is that a .gml files can contain a complete version of a drawing or it can contain updates to a drawing that represent changes to be made. In fact, a .gml file can contain both an original version as well as subsequent updates.

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Importing a GML File

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1. Choose File - Import - Drawing

2. In the Import dialog choose GML files in the Files of type box and browse to the folder containing the GML file to be imported. Double click on the file.

3. The Import GML Data Set dialog will pop open to allow specification of any update files to be processed as part of the import. If importing UK Ordnance Survey MasterMap files, make sure the MasterMap data box is checked. It is checked by default since MasterMap is the only known significant collection of files in GML format.

4. If one or more update files are to be processed, click on the New button in the dialog's toolbar and in the resulting Append dialog double click on the desired update file. Using the New button, add as many update files are desired. The update files will be processed in order from top to bottom, so arrange the list in the order desired using the Move buttons in the dialog's toolbar. Press OK.

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Example

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We will import one of the MasterMap samples provided by the OS on their website at www.cronistalascolonias.com.ar The example is a map of Birmingham that is provided as an "initial data" file and as a "change-only" update file. The initial data file is called www.cronistalascolonias.com.ar ( MB in size) and the update file is called www.cronistalascolonias.com.ar ( KB in size). Given the similarity of names it makes sense for us to avoid confusing the two by keeping the files in different folders, perhaps called "Birmingham initial" and "Birmingham Update."

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The .gz extension indicates the files have been zipped with GnuZIP, which is easily unzipped using WinZip. We routinely use WinZip to unzip .gz files, so our Windows explorer shows the WinZip icon next to the .gz file as seen above.

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When WinZip unzips the files it asks us what extension they should have since the OS apparently does not use extensions for MasterMap files.

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We used the extension .gml to indicate that it is a GML file so if we encounter these files in the future we won't wonder what they are.

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Upon decompression the initial data file, now named www.cronistalascolonias.com.ar, has expanded to MB (!), while the update file, named www.cronistalascolonias.com.ar, has expanded to MB. Note that it would be more efficient to send changes by repeating the entire file in a more efficient GIS format (which would be about the size of the MB zipped original file) than it is to use an unzipped GML changes file.

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We repeat the procedure above to unzip the update file, which will now be called www.cronistalascolonias.com.ar

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To import the .gml file into Manifold we launch Manifold System and then choose File - Import - Drawing using GMLFiles in the Files of type box. We browse over to our initial data .gml file and choose it.

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Manifold opens an Import GML Data Set dialog to allow us to specify any update files desired. We click on the New button to add a new update.

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This opens up an Append dialog that allows us to find and add another .gml file as an update file. We browse over to the folder holding our update file and double click on it.

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The Import GML Data Set dialog is now loaded with the update file we desire. If we click on the update file to highlight it in the Updates pane the other toolbar buttons will be enabled, so we could delete this update or move it up and down in order relative to other update files we might add to the pane. Press OK and the file will import.

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GML format is so inefficient that it can take a very long time to import a GML file, perhaps 20 minutes on an average computer to import a MB GML file. Have patience. Eventually, the result is a drawing and some other components in the project pane.

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If we open the drawing and apply some thematic formatting to area background color (using the theme field) we can see the excellent detail in the OS MasterMap product. Effective use of MasterMap requires some familiarity with the Ordnance Survey's data organization plan. See the OS's web site for documentation.

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Import GML Data Set Dialog Controls

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Versions

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GML entities include version information that allows distinguishing between multiple versions of the same object (that is, between objects with same unique ID assigned by the data provider). When Manifold encounters an object that has already been imported, perhaps as the result of processing an update, it compares the versions of the existing object and the new object, determines which object is more recent and uses the more recent object.

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Notes

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The capability to receive updates was presumably motivated by the immense file sizes required by GML. With updates, the original large file would not have to be downloaded or copied when changes need to be made: a smaller GML file containing changes could be provided so that the original file could be used in conjunction with the smaller changes file. However, GML is so inefficient that just about any other GIS format can contain a refreshed version of the entire data set and still be smaller than only the changes expressed in GML. Manifold does not at present support use of incremental change files, but this capability is expected to be added in future releases of Manifold.

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One way to get around the large file sizes imposed by GML is to do what the Ordnance Survey, does, which is to provide files already zipped in .gz format. The OS suggests that applications might read directly from the zipped file format, but that is not always a good way of dealing with the inefficiencies of GML.

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Because GML files take a very long time to import, if Manifold imported directly from a .gz file in the case of a damaged file users could spend a long time on import finding out that the file was damaged in download or copying. If the .gz file is unzipped first (a rapid process) any errors in the file will be discovered right away before the long process of GML import begins.

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Note that the MB Birmingham sample file in GML format takes tens of minutes to import into Manifold. However, it can be saved as a .map project file requiring only MB of space in a few tens of seconds and thereafter loaded even faster. Compared to efficient GIS formats, GML format is not only 20 times larger, it is also about 60 times slower. For this reason, we recommend that Manifold users import GML files and then save them in more efficient formats, either as Manifold projects or, for large data, within spatial DBMS storage.

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For all the limitations of GML there is much UK data currently available in OS GML form and it is likely that this format will continue in use by the Ordnance Survey. To support UK users of Manifold, www.cronistalascolonias.com.ar expects to work closely with the OS to assure increasing compatibility with OS usage of GML.

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UK users of Ordnance Survey data in GML format are encouraged to send suggestions to www.cronistalascolonias.com.ar for improvements and conveniences they would like to see within enhanced support for GML import by Manifold. See the Contacting www.cronistalascolonias.com.ar topic for information on contributing suggestions.