DXF file is a text file of a special format which contains data about features of AutoCAD draft: names of layers, spatial categories of features and descriptions of features themselves. Besides, a string value can be linked with every feature. From the point of view of ObjectLand this value can be considered as a field value of the table linked with features of the present type. Thus, DXF file can contain not only spatial data, but some table data as well. Detailed description of DXF file format is given in manuals on AutoCAD system.
Essential difference of draft structure of AutoCAD from the structure of ObjectLand map is that there is no notion of a feature type in AutoCAD. A draft is made up of arbitrary number of layers, and every layer can contain features of all spatial categories without differentiation to feature types which is the case in ObjectLand.
A layer of AutoCAD draft can also contain blocks. A block is graphical image composed of different graphic primitives which can be inserted into the draft in several places with indication of block name, binding point, slope angle and scale. ObjectLand does not have a similar structure, however point features can serve as a kind of analogy to blocks, that is why the possibility of importing blocks of DXF file as point features of ObjectLand map is provided. Another possible way of importing blocks is exploding them, that is, processing a block as a set of more simple features.
DXF file consists of a set of groups. Each group is identified by a code which is a positive non-zero integer number. Most values of group codes are reserved for naming system features or characteristics, and codes 3, 4 and 5 – for “…other text or named values”, that is, for specifying some user defined characteristics of a feature. Groups 3 and 4 can contain strings or decimal numbers, group 5 – only hexadecimal numbers. When importing to ObjectLand one of these groups can contain table data linked with features.
It is necessary to perform import setting to set correspondence between spatial categories of features and blocks of DXF file layers on the one hand and feature types in layers of ObjectLand map on the other hand, between text data linked with features and records of linked tables and so on.
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