.. _dgn:

*****************************************************************************
 DGN
*****************************************************************************

File listing
-------------

Data are encapsulated in a single file, usually with the suffix .dgn.

::

	0824t.dgn

Data Access / Connection Method
--------------------------------

*  Access is available in MapServer through OGR.
*  The CONNECTIONTYPE OGR parameter must be used.
*  The path to the dgn file is required, file extension is needed.
*  All types of features in a DGN file are held in one "layer" of 
   data.  The layer is called elements and is the first and only layer.
*  The type of feature to be read from the DGN depends on the TYPE 
   parameter in the map file.
*  DGN files typically contain POINT, LINE, POLYGON and 
   ANNOTATION feature types.
*  DGN files contain "styling" information - how to color and present 
   the data.  This is used, optionally, by specifying the 
   STYLEITEM "AUTO" parameter.

.. note:: 

    DGN files typically use white as a color for their features and therefore
    are not visible on maps with white backgrounds.

OGRINFO Examples
-----------------
Using ogrinfo on a single DGN file:

::

	> ogrinfo /data/dgn/0824t.dgn
	Had to open data source read-only.
	INFO: Open of `0842t.dgn'
	using driver `DGN' successful.
	1: elements

.. note::
    
    No geometry/feature type for the layer is identified because it can be
    multiple types.



DGN files are not really GIS data files.  They evolved from 
drafting formats used by computer aided drafting/design (CADD) programs.  

They carry a few key attributes which are usually consistent across all 
DGN files.  Most of the attributes relate to graphical styling of features 
for map presentation, such as ColorIndex, Style, etc.

Spatial reference system information is not always encoded into DGN files.  
This can be a major problem when trying to adequately reference the DGN 
data in another mapping program.

Measurement units can be a problem.  In some cases the features could 
be located in kilometers or feet even though it is not obvious from 
the output of ogrinfo.  Sometimes the only way to identify or correct 
a problem with units is to open the file in Microstation software.

Using ogrinfo to examine the structure of the file/layer:

::
	
	> ogrinfo -summary /data/dgn/0824t.dgn elements
	INFO: Open of '0824t.dgn'
	using driver 'DGN' successful.

	Layer name: elements
	Geometry: Unknown (any)
	Feature Count: 22685
	Extent: (-513183.050000, 150292.930000) - (-224583.220000, 407463.360000)
	Layer SRS WKT:
	(unknown)
	Type: Integer (2.0)
	Level: Integer (2.0)
	GraphicGroup: Integer (4.0)
	ColorIndex: Integer (3.0)
	Weight: Integer (2.0)
	Style: Integer (1.0)
	EntityNum: Integer (8.0)
	MSLink: Integer (10.0)
	Text: String (0.0)

Map File Example:

::

	LAYER
	  NAME dgn
	  TYPE LINE
	  STATUS DEFAULT
	  CONNECTIONTYPE OGR
	  CONNECTION "dgn/0824t.dgn"
	  STYLEITEM "AUTO"
	  CLASS
	  END
	END # Layer
