How Export To DLG works
This topic applies to ArcInfo only.
Description
Output DLG
The internal feature numbers (cover#) are written as the output digital line graph (DLG) feature numbers.
Coverage topology is saved in the DLG file using conventions that are similar to the way topology is stored in a coverage (for example, polygons are defined in clockwise loops, islands as counterclockwise loops, each feature has a unique identification number, negative numbers for lines indicate reverse directions, and so on).
For each input coverage polygon, the polygon's internal number, its label point location (x,y), the list of arcs that define it, and any major/minor attribute pairs are all saved in an area (A) record in the output DLG.
For each input coverage arc, the internal number of the arc, its from-node and to-node numbers, its coordinates, its left and right polygon numbers, the set of x,y coordinates that define the line, and any major/minor attribute pairs are all saved in a line (L) record in the output DLG.
For each input coverage node, the internal number of the node, its (x,y) location, a list of arcs beginning or terminating at the node, and any major/minor attribute pairs are all saved in a node (N) record in the output DLG.
For each input point coverage point feature, its x,y location and any major/minor attribute pairs are saved in a zero-length Line (L) record. These are referred to as degenerate lines in a DLG. The feature numbers for points will be sequential and will be offset from the last line (L) record written to the file. A node (N) record without the attribute information will also be created for each degenerate line record (in accordance with the DLG format specification).
Coverage Attributes
Coverage To DLG looks for major/minor attribute pairs for line, polygon, and node features in Input Coverage and for point features in Point Coverage.
MAJOR1 MINOR1 MAJOR2 MINOR2 : : MAJORn MINORn
Coverate To DLG first looks for these items in the feature attribute tables created by DLG to Coverage. If these files do not exist, then the appropriate CODE tables will be searched.
Input Coverage Attribute
Table CODE
Table
Major/Minor Attribute Pairs
<in_cover> AAT ACODE ARC attributes written to Line
(L) records in DLG file
<in_cover> PAT PCODE POLY attributes written to Area
(A) records in DLG file
<in_cover> NAT (none) NODE attributes written to Node
(N) records in DLG file
{in_point_cover} PAT XCODE POINT attributes written to Line
(L) records in DLG file
Coverage Coordinates
The x,y coordinates for both input header file control points and coverage features are automatically scaled to fit the DLG coordinate record format unless the Transform Coordinates option is unchecked. With the default Transform Coordinates option checked, all coordinates are translated using the same affine transformation equation. This transformation ensures that the accuracy of coordinates is preserved.
DLG attributes are always integer pairs containing one to six digits. Items must be paired and defined as numeric items. Real numbers are truncated when written to the DLG. Since zero is assumed to be a valid attribute value, -99999 can be used to signify no data. If -99999 is encountered for a Major-code or Minor-code item, then the attribute values are blank in the output DLG, not -99999.
Coverage coordinates are not projected by Export To DLG. The projection parameters, used only to describe the projection system of the coverage coordinates, are saved in the output DLG header records.
The input projection file should be in the same format as required by the Define Projection and Project tools. Projection information will be taken first from the input projection file, then from the input coverage projection definition file.
You can use Define Projection to define projection parameters for input coverage. If coverage projection has not been defined and the input projection file is not used, then no projection information will be written to the output DLG file.
Usually a constant value is subtracted from coverage coordinates, especially y coordinate values in the three- to six-million range (UTM). This is done to preserve accuracy for single-precision coverages. This value is referred to as x-shift or y-shift and should be added back onto the coordinates when they are written to the DLG file. You can specify x-shift and y-shift values several different ways.
The following is the order of precedence that Export To DLG uses when determining which values to use.
- X Shift and Y Shift options.
- Xshift and Yshift parameters in the input projection file.
- Xshift and Yshift parameters in the input coverage (PRJ) file.
- Default value is zero, if not found in any of the above.
The x- and y-shift values are automatically applied to both input header file control points and feature coordinates written to the output DLG file.
Header Files
Header information files are created with your system's text editor. The file may contain any or all of the keywords listed in the next discussion topic.
The default value is used for each keyword when that keyword is not included in the header file. The order of the keywords in the file is not important.
Tics in the input coverage are not written to the output DLG file. To preserve tic locations, use the input header file option and specify the x,y locations for tics as CONTROLPOINTs in the header file.
If CONTROLPOINTs are not specified in the input header file, Export To DLG will use the BND information of the coverage to write values to the header of the DLG file.
Keywords and Descriptions
TEXT {string}—Written to the third DLG record; up to 41 alphanumeric characters. The default is blank.
MAPNAME {string}—Written to the second DLG record; up to 40 alphanumeric characters. The default is blank.
CATEGORY {string}—Name of data category or layer; up to 20 alphanumeric characters. The default is coverage name.
SOURCEDATE {date}{revision_date}{revision_code}—Source date, revision date, and revision code, if any. Dates entered as YYYY. Code is:
P - photo revision I - photo inspection D - digital revision
The default is today's date and no revision code.
RESOLUTION {factor}—Data resolution; the default is 1.0 (real value).
SCALE {denominator}—Map scale; the default is 0 (integer value).
CONTROLPOINT {SW|NW|NE|SE}{lat}{long}{x}{y}—List of four control points with latitude and longitude and corresponding x,y locations of each control point. The latitude and longitude coordinates are in DD, and x,y coordinates are in coverage units.
Any {x_shift}, {y_shift} or {TRANS} values used will also be applied to the control point x,y coordinates before they are written to the DLG file by ARCDLG. The defaults are 0.0 in each field.
SECTIONINDICATOR {indicator_sequence}—Code S, F, or T for the size of the section followed by the sequence number.
LARGESTCONTOUR {interval_unit}—Largest primary contour interval followed by the interval unit (1 = feet, 2 = meters). Present only if two or more primary intervals exist.
LARGESTBATHYMETRIC {interval_unit}—Largest primary bathymetric contour interval followed by the interval unit (1 = feet, 2 = meters, 3 = fathoms). Present only if two or more primary intervals exist.
SMALLESTCONTOUR {interval_unit}—Smallest or only primary contour interval followed by the interval unit (1 = feet, 2 = meters).
SMALLESTBATHYMETRIC {interval_unit}—Smallest or only primary bathymetric contour interval followed by the interval unit (1 = feet, 2 = meters, 3 = fathoms).
HORIZONTALDATUM {datum} - Horizontal Datum of DLG.
0 or blank = NAD 27 (default)
1 = NAD 83
2 = Puerto Rico
3 = Old Hawaiian
4 = Local (Astro)
5 = Guam
VERTICALDATUM {datum} - Vertical Datum of DLG.
0 or blank = NGVD 29 (default)
1 = NAVD 88
2 = Local Mean Sea Level
DLGCENTER {long}{lat}—The longitude and latitude of the zone center point in DMS.
Example
In this example, a coverage named D1COV is converted to a DLG. The coverage projection information defined for this coverage will be used by Export To DLG to add projection parameters to the DLG header records. Coverage coordinates are not projected by Export To DLG. The projection parameters only describe the projection system of the coverage coordinates.
The DLG format (Appendix 2-D of the USGS Standards for DLG) requires that, for coordinate systems that normally have zone numbers, the header section of the DLG must contain both the zone number and the latitude-longitude values for the zone point or the center of the data.
The zone may be specified as a ZONE number in the INPUT section of the projection definition file or as latitude-longitude values for the center of the data in the PARAMETERS section, but the projection file does not support both. To write a DLG that contains both, specify zone number in your projection file and zone point using DLGCENTER keyword in your input header file.
The following is the information in the coverage projection file:
Coordinate System Description Projection UTM Zone 13 Units METERS Datum NAD27 Spheroid CLARKE1866 Xshift 0.0000000000 Yshift 0.0000000000 Parameters
The following header information file named HEADER.FIL will be used to create the appropriate information in the output DLG.
Header file HEADER.FIL MAPNAME DURANGO, CO CATEGORY HYDROGRAPHY SOURCEDATE 1995 RESOLUTION 2.54 SCALE 100000 SECTIONINDICATOR F02 LARGESTCONTOUR 5002 VERTICALDATUM 1 CONTROLPOINT SW 37.250000 -107.750000 256087.53 4125947.50 CONTROLPOINT NW 37.500000 -107.750000 256898.08 4153690.33 CONTROLPOINT NE 37.500000 -107.500000 279000.18 4153071.89 CONTROLPOINT SE 37.250000 -107.500000 278265.94 4125332.02 DLGCENTER -107 37 30 37 22 30
Command Line: arc_arcdlg d1cov example.dlg # utm.prj # # header.fil NO_TRANSFORM
The following lists part of EXAMPLE.DLG, the output DLG file:
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