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GRIDVECTOR (version 1) : AN ARC/INFO AML PROGRAM TO EXTRACT
LINEAR FEATURES FROM A GRAY-SCALE IMAGE OF A PAPER
GEOLOGICAL MAP
by
Feliks Persits
Open-File Report 97-713
This report is preliminary and has not been reviewed for
conformity with U. S. Geological Survey editorial standards.Any use of
trade, product, or firm names is for descriptive purposes only and doesnot
imply endorsement by the U.S. Government.
Although this program has been used by the U. S. Geological
Survey, no warranty,expressed or implied, is made by the USGS as to the
accuracy and functioning of the program and related program material, nor
shall the fact of distribution constitute any such warranty, and no responsibility
is assumed by the USGS in connection therewith.
Figures
Source Code
GRIDVECTOR, an ARC/INFO macro language program, provides
a more effective method of accomplishing the first step of on-screen digitization
of large and complicated geologic maps. The traditional method used is
first, hands-on tracing of selected lines on a transparent mylar sheet.
The mylar is then scanned in black and white mode and the scanned image
converted to lines by the ARC/INFO GRIDLINE or ARCSCAN commands.
Disadvantages of the traditional method are:
- Much more time is needed to prepare drawings on mylar
when features on the original geologic map are very tiny and the number
of polygons is large (e.g., hundreds or thousands).
- Poor accuracy is common because of accidental shifts
of the mylar while tracing lines and because lines may be inadvertently
omitted.
- Adding these omitted features requires rescanning the
mylar and consequently introducing new spatial errors when matching the
new features to the existing data.
- Identifying the same line on the screen and on the original
paper map is time consuming and tedious work.
GRIDVECTOR is designed to trace boundaries of geologic
features from a gray-scale scanned image of a paper geologic map. The advantages
of this method are:
- A gray-scale scanned image is far less expensive than
a color image and the image itself needs much less disk space.
- Even at relatively low resolution (200 dots/inch), most
geological features are retained in the gray-scale image.
- The color separates of the original color map, if available,
can be used as input to GRIDVECTOR to trace colored boundaries.
- Adding newly requested features or correcting existing
ones is simple because the same gray scale image is used.
Scanned images are transformed into a grid, using the
ARC/INFO IMAGEGRID command and georeferenced using the ARC/INFO REGISTER
and RECTIFY commands. The georeferencing is done mainly by using latitude/longitude
crosses or other features with known coordinates as references.
The output of GRIDVECTOR is a line and polygon ARC/INFO
coverage. Here lines are supposed to be one of the following features on
the gray-scale image of the original paper map:
a) the centerlines of the dark, relatively thin, strips
(polygon outlines, faults, etc. on the paper map);
b) boundaries between areas with different gray levels
(different color or pattern on the paper map).
| The method presented in this report extracts lines from
a gray-scale image. These lines (strips of pixels) can be easily seen on
a computer screen in ARCEDIT, when the image has been used as the background.
If the original map is of good quality (i.e., good line contrast and a
small number of different patterns and text) the GRIDVECTOR program can
extract 50-60% of the lines, saving a great amount of handwork and time.
The rest of the digitization, and correction, can be done effectively along
with the geologic attributing in ARCEDIT. This helps avoid errors caused
by separation of these processes in time.
The program is based on the assumption that a dark (mainly
black) line outlines every polygon (geologic feature) on a paper map. Such
a line in gray-scale is represented by a thin strip of pixels with minimum
gray values. Features that can be treated as noise are other dark lines,
such as rivers, and text annotations. To reduce the number of lines and
polygons created because of such noise, the average slope of gray-scale
pseudo relief was calculated. The average gray levels are about the same
on both sides of the dark line representing a river, text, or coordinate
grid. By contrast, a geologic polygon gives a jump in gray level because
of the different color or pattern on either side of the line. The user
has a choice of selecting lines by the minimum of gray values, by average
slope, or by both methods. Before calculation of an average slope, several
consecutive smoothing filters are applied.
The program is based entirely on standard ARC/INFO, ARCSCAN
and ARCEDIT commands. ARC/INFO’s GRID program is limited to a maximum filter
size of 7 cells (pixels). Therefore, the optimal image resolution is about
200 dots/inch, giving a width of 4-6 pixels for dark lines.
The program uses ARC/INFO’s GRIDLINE command to create
a line coverage; the final results are very dependent on the effectiveness
of GRIDLINE.
An easy way to apply the GRIDVECTOR AML is to select parameters
on a relatively small portion of the input grid. The user can then experiment
as many times as necessary, because the program deletes all previously
made files. The goal is to select parameters that allow an optimum output
that satisfactorily filters out the noise without removing too many geological
boundaries.
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