GML Definition and Meaning
GML (or Geography Markup Language according to abbreviationfinder) is an XML- based markup language for modeling, transporting and storing geographic information. It is a computer lingua franca for the management and transfer of information between the different systems that make use of this type of data, such as Geographic Information Systems (GIS). GML was designed from the abstract specification produced by the OpenGIS group (now called the Open Geospatial Consortium, OGC), and from the ISO 19100 series of documents.
GML is extensible, being able to create our own entities from the existing ones by extension, or take those defined in the specification and specialize them for our purposes following the mechanisms of inheritance and structuring in objects.
GML is included in version 1.0 of the “National Information Exchange Model” of the United States of America. GML does not contain specific information on how the displayed data should be displayed. For this, styles are used that are related to GML and are described in other languages such as KML.
- Codifies Coverages: It makes it easier to define the space – time and the range of attributes assigned to that space – time. Very useful to represent layers.
- Encode Observations: Such as photographic shots, flood or temperature measurements. It indicates how the information was obtained, the object of the measurement, instruments and the resulting measurement.
- Encode Metadata: Metadata can be related to an entity as well as to a property.
- Codifies Reference Systems: It has the ability to define datums. It includes the main geocentric reference systems and main projections to apply them to any entity.
- Encodes Time: It allows describing entities in motion or that evolve over time.
- Dictionaries for both things: Allows you to extend the Coordinate Reference Systems (CRS), to localization keywords (SRS) and Measurement Units (UOM).
Advantages of GML
- Automatic Verification of Data Integrity
- GML can be Read by Public or Generic Tools
- GML can be easily edited
- GML can be easily integrated with Non-Spatial data
- Higher quality maps
- They work in browsers without the need to buy client-side software
- Custom map styles
- Editable Maps
- Better query capabilities
- Control over content
- Animated entities
- You don’t have to think only of a web browser
- Chaining of services
Disadvantages of GML
- Large files. GML is text and therefore the information is not optimized from a storage point of view like binary formats are. For this there are two solutions:
- Compress the files in gzip format for transport and storage, forcing the applications to introduce compression / decompression mechanisms. On the other hand, they are simple operations to implement. Compression ratio 5: 1 or higher.
- Binary XML that is supported by both native and open APIs (eg Sax, Dom).
- Cost of adaptation and training to GML technology.
- Difficulties to handle large raster files. Aerial photographs and orthophotos are still stored in the usual raster formats (Tiff …). Although it is planned for the next updates to be able to work with large raster files, even video.