NTDB - National Topographic Database

    File: c:\aacrse\505\ntdb\ntdb.rtf

     2) List of NTDB Entity Codes for the 1: 50 000 National Topographic Data Base
     3) NTDB Standards and Specifications

     Files composing a sample NTDB .DXF Topographic Data Base:
     31go1.rtf - metafile number of entities as points and lines.
     31go1wa.dxf - hydrographic (blue); waterways, coast lines.
     31go1ve.dxf - vegetation (green), woods, orchards.
     31go1ot.dxf - buildings, swamps, dikes, etc.
     31go1co.dxf - contours (yellow) and elevation points (red).
     31go1ro.dxf - roads (magenta).
     31go1.dwg - ACAD drawing file with layers coloured.

     These files are archived in  Earthnt\public\505\ntdb and Users\wrchurch\ntdb


     When obtained from the Ministry the files will be self-expanding zip .exe files, e.g. 31go1wa.exe (131 kb). When expanded, a corresponding .DXF file will be created, e.g. 31go1wa.dxf (1,305 kb). Each .DXF file will contain a set of layers, with each layer named according to the Entity Code of the data on the layer and the DXF codification of the data type (see below).
    Load Autocad and import the DXF file 31go1ro.dxf. Examine the layers in the DXF file, set each layer to some arbitrarily chosen colour, and use MODIFY -> PROPERTIES -> ALL -> to change all the entities on all the layers. In the Properties box click COLOR, and in the Logical Colors box click BYLAYER. Click OK twice. Each object will now have assumed the colour of the layer on which it occurs.
    Freeze the layers, and import the 31go1wa.DXF file by entering the command DXFIN from the command line. (Do not use FILE -> OPEN to add the new DXF file because doing so will force replacement of the current DXF file.)
    Repeat the above procedure to colour the new layers.
    The layer on which any object resides can be determined by clicking the object and noting the layer name that appears in the Layer toolbar window, and the nature of the objects on the layer can be obtained by consulting the list of NTDB Explicit Entity Codes, e.g. 1559 is the code for the layer containing Contour Lines, and 1576 the code for Elevation Points.
    The suffix -1001 is the DXF codification. If the 3rd digit is a '1' it means that the Z value of the point or line being referenced is a valid elevation; if the value is '0', it means that there is no entry or a zero entry for the Z value. The first two digits are pointers to the captured ancillary data (meta-data; see below), and the fourth digit is the geometric representation qualifier. For example, there may be a code 114-1001 and 114-1003 for a 'City Hall' represented on the map as point locations. The former is accurately located, the latter is located only approximately.
    The NTDB Entity code is usually, but not always, a number derived by adding the code number for the entity (e.g. 105 is the code of BUILDING) to the Fixed Attribute Number (e.g. 11) to give the code 116 for a College building.

     Data Specifications:
     A data set consists of all the data found on one NTS map sheet at either the 1:50 000 or the 1 : 250 000 scale.
     The ancillary data of the data set (meta data) contains the data set number (NTS), geographic location, UTM Zone number, contour interval, and number of line and point occurrences per entity.
     Topographic features have both Entity and Geometric Representation features.
     Entities are such things as roads, dams, buildings, etc, and are itemized in the NTDB data dictionary (see the file 31go1.rtf) and are given an Entity code, e.g. roads are identified by the code 993. Entities have attributes which describe the entity, e.g. in the case of roads, the number of lanes, road design, accessibility, etc.) and each attribute is given a code value, e.g. 4 = two lane road. These sets of values are known as an 'occurrence of an entity'. In DXF files, all similar entities with the same Entity code, the same attribute codes, and the same DXF code lie on the same layer. Each individual entity also has an identifier, a sequential number incremented at each occurrence.
     Each topographic feature has a Geometric Representation, of which there are three types: points, lines, and areas. In Autocad 'areas' are not recognized as such, and are represented by closed polylines. The Geometric Representation may possess attributes, namely, an identifier, a position qualifier, and coordinates. The values for the Geometric Representation Qualifier are 'definite' (1) and 'position approximate' for points; 'definite', 'indefinite', 'position approximate', and 'virtual' for line; and 'definite' for ELEVATION (Z). The Geometric Representation Qualifier only applies to points and lines, and not to areas or vertices. As in the case of Entities, giving values to every attribute of a Geometric Representation creates a 'Geometric Representation Occurrence'.