Rotation angle (2D) and quaternion (3D)
Rotation is represented by a value angle, that is counter-clockwise from the X-axis. Angle is a long integer with the lower-order bit equal to .01 seconds. The conversion from angle to degrees is expressed as Degrees=Angle/360000.
Line Elements (Type 3)
Line Elements consist of the header information and design plane coordinates of the line endpoints.
|
Word # |
Type of word |
Desc. : |
no. of bits |
Remarks |
|
{ |
|
|
| |
|
1 -14 |
Elm_hdr |
ehdr |
|
element header |
|
15 -18 |
Disp_hdr |
dhdr |
|
display header |
|
19 -22 |
Point2d |
start |
|
starting point |
|
23 -26 |
Point2d |
end |
|
ending point |
|
} Line_2d |
|
|
| |
Line String (Type 4), Shape (Type 6), Curve (Type 11)
Line string, shape, curve elements are represented similarly. The header information is followed by the number of vertices and then the coordinates of each vertex. A maximum of 101 vertices can be in an element of this type. In a shape, the coordinates of the last vertex must be the same as those of the first vertex. For curves, two extra points at the beginning and end of the vertex list establish the curvature at the ends. Thus a curve can have a maximum of 97 points.
|
Word # |
Type of word |
Desc. : |
no. of bits |
Remarks |
|
{ |
|
|
|
|
|
1 -14 |
Elm_hdr |
ehdr |
|
element header |
|
15 -18 |
Disp_hdr |
dhdr |
|
display header |
|
19 |
short |
numverts |
|
number of vertices |
|
20 -23 |
Point2d |
vertex |
|
point |
|
24 -... |
Point2d |
vertex |
|
point |
|
} Line_String_2d |
|
|
|
|
Text Elements (Type 17)
A text element stores a single line of text.
|
Word # |
Type of word |
Desc. : |
no. of bits |
Remarks |
|
{ |
|
|
|
|
|
1 -14 |
Elm_hdr |
ehdr |
|
element header |
|
15 -18 |
Disp_hdr |
dhdr |
|
display header |
|
19 |
byte |
font |
: 8 |
text font used |
|
19 |
byte |
just |
: 8 |
justification type |
|
20 -21 |
long |
lngthmult |
|
length multiplier |
|
22 -23 |
long |
hghtmult |
|
height multiplier |
|
24 -25 |
long |
rotation |
|
rotation angle |
|
26 -29 |
Point2d |
origin |
|
text string origin |
|
30 |
byte |
numchars |
: 8 |
# of characters |
|
30 |
byte |
edflds |
: 8 |
# enter data fields |
|
31 -... |
char |
string |
: 8 |
characters
|
|
} Text_2d
|
|
|
|
|
Methodology (contd.)
As mentioned earlier, the positional data decoded using the above specifications are in easting and northing, the projected grid coordinates using Polyconic projection on EVEREST 1830 spheroid with reference latitude and longitude as 24o N and 93o E. (These are the parent projection parameters used by the KDM Institute of Petroleum Exploration, in their work.) The cultural data of the state of Tripura obtained this way are shown in Fig. 1.
For universal use of this valuable data, the grid coordinates need to be converted to absolute geographical coordinates that is, latitude and longitude. This has also been achieved in a separate software module. Once that is done, the data can be drawn on a map referred to any spheroid, projection system and reference latitude and longitude. In particular, the geographical coordinates have been converted into rectangular grid coordinates in Lambert Conical projection with reference latitude and longitude as 26o N and 90o E.
Conclusion & Remarks
A methodology and a software package have been developed by which any positional data stored within the Geographical Information System can be deciphered and be independently put to use by any user. One of the greatest advantages of this method is that positional data from any independent source (say, well location, seismic survey lines etc.) can be directly superposed on the data derived by this method. This is because both data may be brought to the same reference plane of absolute latitude and longitude referred to a particular spheroid. This eliminates the use of "rubber sheeting" methods usually employed in GIS to fit external data with the existing spatial data.
The software, currently available in Fortran 77, may easily be ported to any machine supporting the language. It is proposed to make the software platform independent so that it may be run on any computer capable of connecting to the Internet.
Acknowledgement
The author is indebted to Dr.A.K.Balyan, GM(Expln) and Mr.T.K.Dutta, Chief Geologist of ONGC,Calcutta for their support and encouragement to take up this work. Sincere thanks are due to Mr.James Peters, DGM (Geology), Mr.D.S.Mitra and Mr.F.Dotiwala of KDM Institute of Petroleum Exploration for providing the data of Tripura in .DGN files.
Reference
- D.S.Mitra, F.Dotiwala, et al. "PREPARATION OF DIGITAL GEOLOGICAL MAPS FOR NORTH EAST INDIA BASED ON FIELD PARTY SURVEYS USING A GIS APPROACH", ONGC Report, January, 1998.
.
