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Topology in Easy Trace

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Easy Trace enables you to create and verify complicated topological model of vector data. We consider any vectorizer without these abilities to be a tool for time waste.

Easy Trace enables you to create and verify complicated topological model of vector data. We consider any vectorizer without these abilities to be a tool for time waste.


Forming of valid topology in GIS after export of "rough" vector data is doomed to fail (of course, we don't mean projects containing only several map sheets and several independent layers).

If GIS is used just as a Geo-Informational System (rather than vector data viewer), its requirements are very exacting. How can you optimize the route if roads don't form a network? Tiny gaps and crossings are invisible on the screen but there can be hundreds of them...

In any more or less complicated project, digitizing may start only after careful elucidation of topological relations between layers. Necessarily consider the demands of your target system and desirable final data presentation as well as data meaning while tracing! For example, digitized parcels should be represented by polygons with common boundaries as a rule, nodes should be formed at points of road crossings, etc.

Forming of topological model in Easy Trace is based on three foundations:

  1. Setting of topological rules that control relations between layers;
  2. Compiling of job description for operators (that necessary includes the order of digitizing!!!);
  3. Check-up of topology relations applying a set of tests, check-up of the polygonal cover;
Besides the utility searching for topological errors, the program has means for check-up, correction, and optimization of topologically dependant data. These are utilities:
  1. Topology correction - automatic superposition of vertices and "pulling" of hanging line ends together;
  2. Topology optimization - topology optimization by removal of superfluous vertices with preservation of topological connectivity and geometry of small objects;
  3. Search for kinks - search of specific errors in polylines;
  4. Polygon creation - forming of a polygonal cover out of lines of their boundaries;
  5. Polygon check-up - search of topological errors in the polygonal cover;

Topology Rules command

This set of rules allows to entrust tracing tools with automatic topology creation as the rules will be observed automatically at vectorizing.

As control on topological relations is important from the very beginning of digitizing, setting of the rules relates to the stage of project creation. Specify the rules in the Project properties dialog box - Topology of the Project menu.

Formed set of the rules determines behavior of the Topology Editor and tracing tools while digitizing. It controls relations between lines of a particular layer with lines belonging to other layers - the tools may ignore them, form a common vertex, or form a node.

Topology creation at manual tracing

Actually, forming of any particular topological model of vector data comes to a set of operations - snappings and boundary copying. At that, point and linear objects may be snapped to each other in any combination. Easy Trace PRO provides unified snapping control for all the tools. There are three possibilities:

  • compulsory forming of a common vertex;
  • compulsory forming of a node;
  • implementation of the rules of interlayer relations specified in the Project properties dialog box - Topology of the Project menu.

The fourth variant is to ignore the object, i.e. to form a vertex or insertion point where it is specified by the cursor, but it hardly can be called snapping.

Forming of a polygonal cover is totally based on boundary copying although the user may overlook the process as it is automated.

Snapping of vector objects

Use SHIFT, CTRL and the Auto-snap button in the Parameters Bar of the tool (if available) to control ways of snapping. Objects may be snapped at insertion point indication (for point objects) or at forming of a polyline vertex. Similarly, snapping is possible at moving of insertion points or polyline vertices with the Editor.

SHIFT

The key for provides compulsory snapping with forming of a common vertex. Hold down the SHIFT key and click the desirable snapping point with the mouse left button. Snapping is available for:

  • point to point - superposition of insertion points of the objects;
  • point to line - forming of a new vertex on the line (lines) at the insertion point;
  • line to point - precise attachment of a line vertex to the insertion point;
  • line to line - forming of a common vertex on all the lines at the specified point.

CTRL

the key for provides compulsory snapping with forming of a node. Hold down the CTRL key and click the desirable snapping point with the mouse left button. Snapping is available for:

  • point to point - superposition of insertion points of the objects;
  • point to line - forming of a node on the line (lines) at the insertion point;
  • line to point - forming of a node on the line at the insertion point;
  • line to line - forming of a node on all the lines at the specified point.

Auto-snap

The option is available in the Parameters Bar of the tools intended for creation of point objects (i.e., point, block, circle, and text). When activated, it forms a common vertex or a node automatically depending on the topology rules specified in the project. SHIFT and CTRL keys modify the effect of the option.

Pressing of SHIFT or CTRL changes the form of cursor and a message appears in the Status Bar.

Another way to form a node at the given point (e.g., at digitizing of power line segments between supports) is the Finish and start command in the submenu of tracers (right click anywhere within the working area to open the submenu). Use the command if you want the endpoint of one line to be the initial point of another line.

Polyline copying

Parts of two and more polylines should coincide sometimes (for example - escarps along road boundaries). Complete coincidence of linear objects belonging to different layers also may be required. The following means are provided for this purpose:

Copy a polyline part

Select the Copy a polyline segment command in the tracer submenu, then click the initial and the end points of the line part you want to copy. If you try to copy a part of a closed line, the program automatically chooses the shortest of two possible ways between the specified points. If you need the long arc, click several additional points on it;

Polyline doubling

if you want to create an exact copy of an object on another layer, select the object with the Editor and apply the Duplicate command of the editor submenu.

Polygon forming in Easy Trace

Simple and effective methods of polygonal cover forming constitute a significant advantage of Easy Trace. They are based on the Autoclose function first of all, together with polygon cutting and joining.

Autoclosing

The process may be considered by the example of a square in the middle of a chess-board, contiguous with eight other squares. It is enough to "take" corresponding parts of boundaries of four squares to form the boundary of the one under consideration. That is how tracing tools form polygons in the Autoclosing mode.

To start the mode, click the button in the Parameters Bar of a tracing tool. The mode is available for polygonal layers only.

To permit polygon forming on a layer, put the flag (red rhomb) before its name in the layer list of the project, like that: . Otherwise autoclosing will be inaccessible!

If both ends of a line are snapped to other polylines and the Autoclose mode is on, the Tracer tries to close the line, i.e. to form a polygon applying parts of other polylines and polygon boundaries.

If an area is already bordered by linear objects (e.g. a parcel - by lines of fences), it is enough to specify two neighboring points in any of the lines with a Tracer to form a polygon.

The program does not distinguish linear objects and boundaries of other polygons at search of the route for polygon closing. It is convenient sometimes to apply temporal bridges and close polygons when you puzzle out the "polygonal mosaic".

Cutting of polygons

If you seize up an adjacent polygon at polygon closing by accident, you needn't click UNDO and repeat the operation. It is much easier to cut off the surplus part with the help of the same tracing tool! Just draw the cutting line snapped to the polygon boundaries (the Autoclose mode should be on of course) to divide the polygon in two.

The program repeats autoclosing of the initial polygon and subtracts a new one from it.

Polygon closing always occurs by the shortest route. Resulting polygon belongs to the current layer of the Tracer. So, if a big polygon seized up several small ones from another layer, change the current layer and cut them off!

You may also cut out polygons - "islands". For this purpose one should apply the Editor . Select the big polygon first, then start the cutting mode and click the island. That's all.

Polygon joining

Polygon closing occurs sometimes via an "additional" crossing line. To join the parts, take the Editor again . Select one polygon, then start the joining mode and click the second polygon. If they belong to different layers, the resulting polygon will be attributed to the layer of the first one.

Editing of polygon boundaries

Boundaries between polygons are made of two lines at least but may be edited as ordinary polylines if you apply the specialized Topology Editor . The commands are the same: left click to add a vertex, right click to delete a vertex. Besides, you may select an existent vertex with the mouse left button and move it.

Topology creation at semiautomatic tracing

Everything written above is true not for manual only but also for semi-automatic tracing mode. At that we recommend to select the first point of a polygon you want to vectorize on any existent polyline. The next vertex determines direction of raster line tracing.

At crossings with vector lines the Tracer forms a common vertex, a node, or ignores the line and continues tracing according to the specified set of topology rules. If a node is formed, line tracing breaks.

If the Autoclose option is on, the Tracer tries to close the polygon at crossing with vector lines. When succeeds, line tracing breaks.

The Create Polygons utility may be attributed to automatic operations of topology creation. When applied, any crossing of polylines belonging to the selected layers may serve as a common vertex at polygon forming. For example, it is enough to draw a frame and cross the field with vertical and horizontal lines to form a polygonal "chess-board".

Source lines remain unchanged at polygon forming; polygons may be formed on another (specified) layer.

Verification of topology

The Topology Check-up command of the Utilities menu allows to estimate accuracy of vector structure applying a number of criteria. Combination of layers to be tested and the rules to be applied forms a topology test . You may give it a name and save for further applying.

As a rule, most experienced operator compiles the set of tests and specifies the order of their applying. We recommend to name the tests in a comprehensible way ("Roads", "Rivers", etc.). It is advisable to prepare tests before distribution of projects among operators, for example at fulfillment of a pilot project. Then the once created set of tests will be inherited by all prototype-based projects or at subproject creation .

If you improve the set of tests in the course of real work, you may deliver it to operators applying the Import strategies command.

The utility of topology verification makes vector objects of a special kind - error marks on the _ERRORS_ layer. The marks indicate location and type of topological errors. Hot keys are provided for the Editor and some other tools to simplify navigation from one error mark to another: F - "jump to the next mark " and V - "return to the previous mark". An error mark may be deleted as any vector object, at that the object where the error was found automatically becomes selected for editing.

Don't postpone topology verification till the end of the project. We recommend to carry it out after every digitizing session. Besides, it's extremely useful to repeat the operation after correction of the errors - "every last found error is the next to last" and you could even make new ones in the process of editing.

To verify topological correctness of the polygonal cover, apply the specialized Check Polygon Cover utility. In that case, error marks are polygons drawn by the utility (on the _ERRORS_ layer of course) around holes and overlappings in the cover.

Topology Check-up utility

This utility verifies correctness of vector data topology created in your project applying different estimation criteria.

The following topological errors may be found and marked:

Self-intersection - crossing of two segments belonging to the same polyline. "Cross" intersection - crossing of two polylines without a common vertex or a node at the point of intersection.
Vertex intersection - crossing of two and more polylines with a common vertex at the point of intersection whereas a node is required. For point objects - superposition of the insertion point and a polyline vertex but not a node. Dangling node - end of an unclosed polyline, which does not coincide with any vertex of any other polyline or a point object's insertion point.
Unclosed polyline - initial and the terminal line vertices do not coincide. Partial overlapping - overlapping of segments belonging to different polylines (i.e. two neighboring vertices at least) .
Pseudo-node - superposition of end (initial) points of two (necessarily!) unclosed polylines attributed to the same layer. For point objects - superposition of insertion points of objects attributed to the same layer. Vertex duplication - superposition of two adjacent vertices of the same polyline (executed at search of self-intersections).
Node with degree>= 3 - superposition of end (initial) points of three and more unclosed polylines attributed to the same layer.

Additional options of verification criteria allow to diminish number of possible fictitious errors.

The Skip single ones option can be set for the Dangling nodes criterion. Check it, if one dangling node is permissible for lines of the layer to be verified (e.g., river tributaries).

The Pseudo-nodes criterion is supplemented with the with equal attributes option. Transfer from an asphalted road to dirt one will not be marked if all parts of the line on the "Roads" layer have correct attributes.

Test forming and data verification

You need a set of topology tests to check your vector data (a single test may be sufficient for a very simple material only!). It is not so difficult to form the tests if they are not ready yet - just tick off the layer (layers) and the criteria you want to apply for verification, then save the test, and that's all. But do not yield to temptation to include just a little more layers and criteria - the result will disappointing...

Recollect vector model requirements concerning the selected layer - inter reaction with other layers at line crossings, presence of point objects, attributes, etc.

Example. Specification of the "Roads" layer: nodes at all crossroads, every segment of the road has an attribute indicating pavement type, roads are connected with objects of the layers "Constructions" (bridges, culverts, etc.) and "Settlement Borders". Besides, objects of the "Rivers" layer (rivers, brooks, canals ...) form common vertices with objects of the "Roads" layer at crossing.

The first idea that comes to mind is to check the layer applying the Self-intersection, Cross-intersection and Vertex intersection criteria. The first of them is evident, and two next are forbidden by the demand "nodes at all crossroads".

Now, let us consider Dangling nodes. They are not errors for the roads "to nowhere", but inadmissible for other road lines. Applying of the criterion will be useful but it will generate marks of fictitious errors at the same time. It is better to form an individual test with this criterion only and to run it once or twice after all other checks and corrections.

If there are many marks of false errors in the project, act as following to avoid second review after editing. Note the first mark when you start editing (for example, draw a circle around in on the "0" layer). Then navigate from mark to mark and correct topological errors but DELETE ONLY MARKS OF REAL ERRORS. All the marks are looked through when you returned to the encircled one. Open the list of object layers and write down the number of objects on the _ERRORS_ layer, then run the test again. If the number of found errors is equal to the number you have noted, all real errors are already corrected.

The Pseudo-nodes criterion is quite applicable. Roads should not be divided in small segments for no reason. On the other hand, the "pavement type" attribute is specified in the performance specification, and thus pseudo-nodes divide road lines into segments with different values of this attribute. But this contradiction is easy to resolve! Neighboring segments have different attributes, so we tick off the option with equal attributes.

It is useful to make sure first that all polylines belonging to the "Roads" layer have attributes. Switch off all other layers and select the Object without attributes mode in the View Modes bar (the button of the mode is in the lower right corner of the working window). Objects with attributes become grey in this mode, and ones without attributes will be yellow.

Two remaining criterion - Unclosed polyline and Nodes with degree>= 3 are obviously inapplicable. So, the first test is ready, and we save it.

Next step - check of relations between the "Roads" layer and the layers it has interconnections with. Remember that the utility does not tell the difference between the layer under consideration and all other layers selected for the test, but applies all selected criterion to all selected layers. One should be very attentive to avoid revealing of numerous fictitious errors!

Polylines of the "Constructions" layer either cross polylines of roads (culverts, pedestrian overpasses) or partly coincide with them (bridges). Lines of water objects form common vertices with roads and may partly coincide with "Constructions" (culverts).

So, we select three layers: Roads, Constructions, and Rivers, then tick off the Cross-intersection criterion and determine its options. It is reasonable to switch off the option On the same layer (we are verifying crossings between different layers) and switch on the Skip duplicate segments (otherwise all superpositions of roads and bridges, rivers and culverts will be marked as errors). Other criteria are unsuitable for this group of vector layers, so the test is ready for saving.

Similarly we can check correctness of contacts between objects on the layers "Roads" and "Settlement Borders" (i.e. presence of common vertices). We select these layers and the Cross-intersection criterion again, switch off both options - On the same layer and Skip duplicate segments (as no overlapping is permitted for the objects). The test is also ready for saving.

That is all - we have compiled 4 tests for the "Roads" layer (including one, that looks for dangling nodes and inevitably generates marks of fictitious errors). It is time to make sure that they mark the errors we want to find and nothing superfluous.

If a test detects a lot of fictitious errors and you do not want to exclude any criterion, divide the test in two or more, i.e. diminish number of layers in the test and specify options of the selected criterions more carefully. Big number of tests does not cause any problem unlike detection of false errors.

When the tests are over, you will see a window with information about all found errors. Close it for automatic start of the vector data editing mode . The working area automatically shifts to the first found error and the error mark itself becomes current (selected with the Editor). If you delete the mark with the Del key (or Delete command), the object - source of the error becomes selected for editing.

At utility restart, the program searches for error marks remaining in the project. If they are found you'll be asked if you want to delete them. Error marks are vector objects of a special kind. They are saved in the project between sessions until you remove them. Error marks are not exported.

Hot keys are provided to simplify navigation from one error mark to another: F - "jump to the next mark " and V - "return to the previous mark" (or commands of the Editor's submenu and conformably).

Topology Correction utility

Unlike the human being, the computer does not understand "almost". Vector data that look perfectly may contain a lot of mistakes - nodes "broken apart", small gaps between lines, etc."Almost coincide" does not work at topological model creation.

If "ignore the line" option is selected as a topology rule of relations between topologically interconnected layers, the model simply "goes to pieces". However, such serious errors are improbable at digitizing in Easy Trace. More often we face them at editing data generated by other programs.

Not long ago, one of ArcView versions "took" polygons at viewing and "forgot to put them back into place". The result was invisible but quickly ruined the polygonal cover.

Another well known situation when automatic correction of vector data is desirable - "backfitting" of material after automatic vectorization.

The Topology Correction utility simplifies improvement of such data (making the number of manual corrections tens times less sometimes!). It "pulls together" nodes or common vertices within the user-specified snap distance. Besides, it may delete gaps between line ends.

The utility resembles topology verification in many respects - you select a set of layers, a set of conditions and additional options; you may save the created set. However, the layers are not equal for the operation - you may select a layer for alternations (marked with the symbol) or as a key one (marked with the ).

Vertices belonging to the layer(s) selected for alternations may be moved or deleted.

Key layer - contains vertices, which will not be moved but considered at correction, i.e. vertices belonging to other selected layers may be pulled to them. Don't confuse the key layer with a "frozen" one. New vertices MAY BE CREATED on the key layer! Key layers are considered first and form the "magnet", which attracts vertices of layers selected for editing.

Example. A gap between an isoline and the frame (key layer!) will be deleted and a common vertex will be formed. At that, disalignment of the frame (due to finite accuracy) will be only thousandths of the image pixel.

The Consider interaction between objects of group allows to select a variant of interrelations of objects:

  • inter pares;
  • within one layer;
  • by reference layers.
Superposition of vertices of several polylines Vertex generation at crossings or gaps between two or more polylines

You may cancel correction results by the UNDO command.

Limitations of the utility

If the number of vertices of a resulting polyline exceeds 8190, the line will not be alternated but copied to the layer of errors and a corresponding message will be generated.

Topology Optimization utility

Easy Trace is often being used for correction of data previously vectorized and loaded to a GIS. Necessity of such correction arises from the very nature of manual digitizing (inevitable sometimes). It is much easier for the operator to put several superfluous vertices than to find real point of line inflection in the image.

As a result, the project is overladen with excess data and its quality decreases. Your target GIS becomes irritaitingly slow.

"Lightening" of vector data is a complicated task if the polygon cover is already formed. Any vertex of any polyline may provide contact with other objects, and can not be deleted or moved without their alternation. Besides, it is difficult to tell the vertices that may be corrected from ones essential for shape preservation.

The Topology Optimization utility is elaborated for this purpose. It deletes unnecessary vertices from the specified layers but remains untouched topological integrity of polygonal, point, and linear objects.

Note! If you are afraid to damage some layers by optimization but want to keep their topological relations with other layers, just "freeze" them before the operation.

If you don't like the result of topology optimization, select UNDO and improve parameters of the operation.

Example of use:

Vector data before...
... and after optimization.

Search for kinks utility

Lightning-like turn is a specific breaking of line form occurring at automatic and semi-automatic digitizing of badly eroded raster lines. It is particularly frequent if you apply the Broken Line Tracer and specify low value of Approximation precision.

Kink - the mark indicating the place of the error

This error may also occur at automatic vectorization of color subject layers extracted from topographical maps. Whatever the case, raster filtering before digitizing reduces possibility to face the problem.

Looking for sharp turns, the program considers combination of two conditions - small length of neighboring segments and small angle between them.

Delete Error Marks utility

Some tests generate a lot of fictitious errors sometimes and their marks make difficulties for vector data review and correction. The best way out is to improve such a test or divide it in two, which would not find false errors.

But fictitious errors may result from the very structure of vector data. In that case you may try to weed them out with the help of the Delete Error Marks utility. It is a success usually as topology verification tests consist of several layers and several verification criteria as a rule, and the utility enables you to select a PARTICULAR type of error mark.

The utility deals with all kinds of error marks generated at vector data check-up, correction, joining and Z-errors.

Create Polygons utility

A considerable part of vector data is inevitably duplicated in any project containing a polygonal cover. These are polygon boundaries first of all, often resulting from a linear object - a road, a line, a fence, or simply a line where one type of road surface or green plantations is substituted by another.

Although such "substitution lines" do not correspond to real objects and indicate only polygon boundaries, they are represented in source material and often required in final maps, particularly at printing.

As it is difficult to use different line types for different parts of a polygon boundary, these lines should be attributed to a special Cosmetic layer. So, "substitution lines" are equal in rights with other linear vector objects.

And now the point of the approach. It is much easier to form polygons out of vectorized lines than to extract such lines from polygons. Even better - polygons may be formed automatically if the lines that border them are digitized accurately. The Create Polygons utility is intended for it.

A certain order should be necessary considered at vectorization. For example, the polygon of the lake should be drawn first to bind lines of rivers to. It means that the utility of automatic polygon cover forming deals with a mixture of both polygonal and linear objects. Unfortunately, new polygons will be formed on top of existing ones, but they will be automatically found by the Check Polygon Cover utility. Deletion of superfluous and merging of adjacent polygons are simple operations that will not take much time.

Check Polygon Cover utility

Automatically formed polygonal cover inevitably has some errors. They are caused by different reasons, by there are only two types of them - holes and overlappings. The types don't require explanation.

Holes Overlappings

Holes and overlappings are typical of manual polygon forming if the Autoclose is off. They usually occur due to inaccurate copying of polygon boundaries, so we recommend to remain this job for the Tracer. If the tool refuses to close a polygon (it happens sometimes), specify intermediate closing lines and do it in parts. Assembling of parts into one polygon is a trivial operation- select one of the them with the Editor , select the command and click others. Perhaps, you will want to delete several vertices on adjacent boundaries. Use only the Topology Editor for this purpose! Editing of a polygonal cover with the Editor of vector entities is a source of errors!

Just as other checking utilities, this one indicates location of errors but generates new polygons on the _ERRORS_ layer rather than error marks. Boundaries on the new-created polygons outlines holes, overlappings, and "feelers" in the polygon cover.

Apply F (next) and V (previous) or commands of Editors' submenu ( and correspondingly) for navigation from a corrected error to the next/previous one. Press DEL to delete selected polygons - error marks.

It is not necessarily to create a polygon cover on the entire project area at once - you may check it in parts. Remember, the utility is a resource-consumpting one. The time it takes increases by the square-law at increase of number of polygons. Thus, we recommend to check and correct projects before merging into a big cover.

Perhaps, you will prefer to check blocks of several projects if number of polygons and computer capacity allow. Verification of the entire polygonal cover may take tens of hours!

As verification of the polygonal cover involves all edges of all polygons, it will take less time if you run Topology Optimization first.

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