|Effective joining of line segments after automatic tracing
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|Автор:||trace98 [ Чт авг 20, 2009 6:39 pm ]|
|Заголовок сообщения:||Effective joining of line segments after automatic tracing|
A few words first about application methods and abilities of the Breakup Joinig utility first:
1. Crosswise joining of line fragments (forming of intersections) is FORBIDDEN
2. To control the process, you may specify the distance to search for line continuation within, and the minimal angle of line convergence
3. The utility can treat short lines as undirected points
4. Line joining is forbidden across lines belonging to “barrier layers”
5. Line end snapped to a line of the current (changeable) or a “barrier” layer does not participate in joining
About vector data formed by automatic tracing
As a rule, there are a lot of short lines in such vector data. Every Т-like or Х-like line joining consists of 3 or 4 segments correspondingly. Long lines are broken by joining of the smallest branches. And THAT’S RIGHT!
Any tiny branch can be the only remnant of a snapped line, for example cleaned out at map drawing to provide a whitespace for inscription, or after crossing with a line of another color. The question is how to separate “useful” segments from “harmful” rubbish.
About the method of joining now:
1. The first (short-distance) Breakup Joining.
This operation “sews together” most reliable fragments of long lines. It may close up gaps in a hatch line, or gaps at sites of crossing with lines of other colors, or gaps remained after “rasterization” of the grid and some other objects (breakthroughs in relief, etc.).
- Specify barrier layers (for example, the layers Frame and Relief_features at isoline joining)
- Radius: 1.2-1.5 typical gaps in hatch lines
- Treat lines as points: (Yes – if there are many «pale» fragments directed across initial lines)
- Minimal convergence angle: 140-160 deg. (i.e. to join only straight or smooth line parts)
2. The first (cautious) Raw Line Filtering.
The task is to delete APPARENT rubbish + to sew together lines at sites of already deleted branches, bridges, forks, loops, etc. The approach is quite simple – to delete all the objects that KNOWINGLY are not fragments of vector lines – numerous loops in vectorized text inscriptions, sites of isoline conglutination, and others.
”Caution” means an attempt to preserve all «correct» line fragments for further joining.
П Selection of filtering parameters is easy:
- Select the feature to process (branch, bridge, adhesion …);
- Click the R button ("Reset") to set minimal parameter values for this feature;
- Select filtering parameters by SPECIFYING EXAMPLES OF THE FEATURE IN THE SCREEN;
- Run the utility …
- … and run it again (until the program stops to find features that become available after previous filtering).
3. Topology correction
The task is to «pull» and «fix» ends of «correct» lines. These may be for example attaching of contour lines to the frame, snap of river lines to polygons of lakes, etc. The operation excludes «fixed» line ends from the subsequent «strict» breakup joining (plus forming of the correct topology model as a bonus).
"Topology Correction" utility fulfills this task.
4. The second (strict) Breakup Joining
The operation must delete long gaps in lines and DECREASE the amount of rubbish lines by SEWING THEM TOGETHER.
Radius – maximal;
Convergence angle – negative (to sew divergent lines together)
It closes up gaps that looked like hopeless. Besides, defects and rubbish segments become more evident and easier to delete. For example, tens rubbish fragments of a former inscription joined by the operation may be deleted later by one touch of Vector Eraser in the “red” mode.
5. The second (strict) Raw Line Filtering
It differs from the first filtering by much more “aggressive” selection of parameters. Nothing to be afraid – everything that could be joined automatically is already joined …
6. Manual correction at sites of joining and line shape improvement.
Use the Inspector tool (see Edit -> Inspector) for guaranteed examination of the entire project area.
Editing tools – Vector Eraser and Camber Editor, plus manual joining with Editor in rare instances.
Apply the tools to:
- trim bent line ends that hinder correct automatic joining of lines;
- cut off and delete attached «rubbish» objects
- delete «humps» and correct the shape of BADLY DEFECTIVE line parts;
- Manual joining should be used seldom in most complicated sites.
ATTENTION!!!!!!!!!!!! All the described operations should be applied to vector data received after automatic tracing. This vector is «dense» - approximately one vertex per one pixel of the image, and thus has superfluous information about line shape. It enables AUTOMATIC DELETION OF MOST LINE SHAPE DEFECTS AT OPTIMIZATION.
You may copy the vector layer, optimize it, and compare with initial lines to understand which defects require manual correction. IT WILL HELP TO AVOID LONG MANUAL LABOR WHERE THE UTILITY CAN MANAGE
7. inal joining and topology correction
Start the utility with the same parameters once again to sew together lines where bent line ends were deleted manually.
8. ONLY NOW it is time to run the Line Form Optimization utility.
If you don’t like the result, click «Undo» and repeat optimization with another parameters.
9. Control of topological connectedness
This operation will find “hanging” line ends that are not attached to any object and nodes of high order (e.g. tributaries breaking the line of the main channel). Information from other layers may be used at topology control – search of forbidden intersections.
"Topology Check-up" utility fulfills this task.
10. Final correction of found topology errors
Most of the operations described above take from tens seconds to 1-2 minutes. The only exception is item 6 – it’s a manual work after all. Anyway, time saving may be tens times and more in comparison with semiautomatic or (never again!) manual vectorizing, for data-rich maps especially. The more objects – the more is time gain.
An example of the technology is represented in the video Vectorizing of vegetation type polygons in Easy Trace Pro»
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