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Vegetation digitization

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Input of vegetation data (vectorizing of point symbols and dotted lines of vegetation types’ boundaries, polygon assembling and input of their attributes) takes 3 – 3.5 hours per one sheet of a topographic 1:50 000 map.

This instruction was developed for vectorizing of slides of permanent storage (DPH) that constitute the base of GLONAS navigational maps. It primarily reflects the method used for recognition of linear, point, and polygonal objects in the maps and their topologically correct mutual adjustment.

As the project performance specification stipulated SXF format of final data, the instruction comprises some operations specific for Panorama GIS. These operations are terminal in the process stepwise description and do not hinder one from use the method at data preparing for any other GIS.

The main advantage of the method is vectorizing time reduction from several tens to several man-hours.

Input of vegetation data (vectorizing of point symbols and dotted lines of vegetation types’ boundaries, polygon assembling and input of their attributes) takes 3 – 3.5 hours per one sheet of a topographic 1:50 000 map.

Strategies of all utilities, the set of project layers, etc. may be taken from the project-prototype (download).

Main steps of the process:

  1. Recognition of vegetation symbols.
  2. Correction of vegetation symbols.
  3. Automatic forming of dotted lines.
  4. Check-up and quick manual correction. Separation from roads.
  5. Snap of vegetation boundaries to each other and lines of other layers.
  6. Check-up and correction.
  7. Ascription of dotted lines to the target layer and input of their attributes.
  8. Check-up of topological connectedness.
  9. Polygon assembling.
  10. Input of polygons' attributes.
  11. Final topology check-up.

Step 1. Recognition of vegetation symbols

Vegetation symbols are always present in topographical maps. There may be from several hundreds to several thousands of them in a map sheet. According to GIS “Map” requirements, all of them should be vectorized as points and some substituted later by blocks. It is clear that manual input of numerous symbols is extremely time-consumimg.

Vectorizing effort can be significantly diminished by use of the Autodetect Topo-Symbols utility. It automatically recognizes topological symbols on the base of user-specified patterns, puts recognized objects to target layers and assigns them an attribute value in accordance with the qualifier. Recognition patterns may be combined into strategies, i.e. saved sets of parameters. The utility can also delete images of recognized objects from the map to simplify further processing. It is useful at recognition of similar topo-symbols.

The prototype we provide contains ready patterns (masks) and recognition strategies. It is easy to edit them if necessary or to add new patterns by symbol pointing on the screen.

An intermediate step of recognition (the utility has marked recognized symbols)
Recognition result
  1. Recognize point symbols of vegetation applying the "Recognition of vegetation symbols" strategy of the Recognize Topo-Symbols utility. Make sure that the "Rasterize symbols"option is off.
  2. It is impossible to distinguish between the symbols of bushes and shrubwood at this recognition stage as points in the former may be situated anyhow around the symbol center. That’s why central elements of all bushes together with shrubwood signs were recognized and attributed to the “Auto-bushes” vector layer. It is necessary to recognize points in bushes to tell one symbol from another. Recognize points in the image applying the "Recognition of points" strategy of the Recognize Topo-Symbols utility. Make sure that the "Rasterize symbols"option is off.
  3. All points in the image are recognized. Apply the Distinguish Bushes from Trees utility now. All points belonging to bush symbols will be deleted and remaining ones belong to dotted lines that show vegetation type boundaries.
  4. Symbols of bushes are recognized and ascribed to the "VEGETATION_pnt" layer with assignment of the corresponding attribute value. Now, select remaining points with the Group Editor, change their layer for "FILLING SYMBOLS_pnt" and assign them the "Shrubwood" attribute value.

Step 2. Correction of vegetation symbols

Unfortunately, automatic detection of topo-symbols is not perfect. The utility recognizes some symbols by mistake and does not recognize badly distorted symbols at all.

Final debugging remains manual. Use the Vector Eraser tool to delete irrelevant symbols and point cloning to add missing ones. Anyway, it is better to edit data for half an hour than spend the entire working day for manual input.

  1. Review the project field systematically applying Inspector.
  2. Delete symbols recognized by mistake with Eraser. Add missing vegetation symbols with the Point tool; for quick selection of object type specify any similar object with the mouse right button.

Step 3. Automatic forming of dotted lines

Vegetation type boundaries in topographic maps are usually represented by rather distinct dotted lines. They seem to be very suitable for automatic tracing but the operation is often complicated in practice.

Even manual tracing of the boundaries is problematic at sites where dots are hidden by the map grid. Besides, some dots in the map may be elements of topographical symbols rather than dotted lines.

Nevertheless, the Autodetect Dot Lines utility generates smooth vector boundaries of vegetation types out of preliminary recognized dots.

Before line forming
... After line forming
  1. Generate dotted lines out of dots of the "Points" layer applying the Autodetect Dot Lines utility.

Step 4. Check-up and quick manual correction. Separation from roads

Automatically generated boundaries of vegetation require manual debugging. A quick review of the project field enables you to find and delete gross shape defects, erroneous line joining, and "rubbish objects".

Remember that this review should be quick indeed. There is no sense to seek after absolutely correct boundaries at this stage. Your objective is to simplify subsequent data processing with other utilities.

Dotted lines represent not only vegetation boundaries but winter roads as well. The utility does not tell the difference between these objects. Thus, the second objective of manual processing is to separate them and to restore road centre lines. To do it quickly, correct the line representing one side of the road and move it with the Shift / Resize Polyline tool.

  1. Review the material briefly, without using of the Inspector tool. Switch on the HYDROGRAPHY layer and the source image with dotted lines. The objective is to check-up the structure and to join line segments.
  2. Pay special attention to winter roads. Restore one side of the road and move it to the center with the Shift / Resize Polyline tool (hot key - R). Change the layer for "ROADNET_lin" and assign the attribute value "Winter roads".
  3. Delete the second side of the road.

Step 5. Snap vegetation boundaries to each other and lines of other layers

Apply the Topology Correction utility after preliminary manual data improvement. The utility will snap lines of vegetation boundaries to objects of other layers (lakes, rivers, winter roads) and will adjust objects within the layer.

Adjustment of vegetation boundaries with other objects is a compulsory stage at forming of the integrate vegetation coverage.

Before line snap
After line snap

 

  1. Run the Topology Correction utility to snap line ends to the project frame, lines of the hydrographical net, roads, and with each other.

Step 6. Check-up and correction

Careful review of the entire project ensures correct forming of vegetation type boundaries. Pay special attention to points where dotted lines cross or join each other. The latter are usually represented by 3-degree nodes after automatic recognition. Joining position may require some manual correction.

As pseudo-nodes and 3-degree nodes are errors for GIS "Map", run in turn the Breakup Joining utility (“Dotted lines pseudo-nodes” strategy) and the Raw Line Filtering utility ("Т-joints vegetation" strategy) to combine two lines into one at every 3-degree node.

  1. Review the project field applying the Inspector tool to correct line form and to solve ambiguous situations.
  2. Snap line ends to objects belonging to other layers
  3. Unite lines at pseudo-nodes applying the Breakup Joining utility.
  4. Correct sites where threу join applying the "Т-joints vegetation" strategy of the Raw Line Filtering utility.

Step 7. Ascription of dotted lines to the target layer and input of their attributes

Dotted lines – vegetation boundaries are independent objects in GIS “Panorama”. They should be ascribed to the "OBJECTS FOR MAP PUBLISHING_lin" layer and receive the "CONTOURS OF GROWTH, SOILS" attribute.

  1. Select dotted lines with the Group Editor, change their layer for " OBJECTS FOR MAP PUBLISHING _lin" and input the attribute " CONTOURS OF GROWTH, SOILS "

Step 8. Check-up of topological connectedness

All lines used at polygon assembling should form a closed arc-and-node model to ensure correct creation of the polygonal coverage. To check the model, run the Topology Check-up utility with the "Check of reference lines for polygon forming" strategy.

Correct all detected mistakes and run the check once more.

  1. Use the "Check of reference lines for polygon forming" strategy of the Topology Check-up utility to find dangling nodes.
  2. Review and correct the errors. Snap lines to other vector objects. Delete short segments.

Step 9. Polygon assembling

It is enough to run the Create Polygons utility for polygon assembling when the arc-and-node model is ready.

Unfortunately, the Create Polygons utility does not support strategies yet. So make sure that all layers participating in polygon forming are specified. As a rule, these are layers of linear and polygonal hydrographical objects, roads, relief, and frame. And the layer of vegetation boundaries of course!

Before polygon assembling
After assembling
  1. Apply the Create Polygons utility to form polygons of vegetation types.

Step 10. Input of polygons' attributes

Make the color image visible at input of polygons’ attributes. We also recommend to switch on the "Fill Polygons" and "Thematic Displaying" view modes – it will help to tell polygons with and without attributes from each other easily.

Use the Attributes by Sample tool to copy all attributes of the specified object and assign them to another one.

Before input of polygons' attributes
After input
  1. Input attributes of large polygons.
  2. Input attributes of small polygons. It is convenient to review the material and input attributes of one-type polygons with the Attributes by Sample tool.
  3. To avoid individual search for every omitted polygon, select and mark all polygons without attributes at once with the Group Editor, and then apply quick navigation between marked objects.

Step 11. Final topology check-up

It remains only to check-up topology again and to correct detected errors. Strangely enough but there will be some most likely. As well as the mean to find them.

Check-up strategies are applicable for an individual map sheet and several merged sheets, up to the entire map.

  1. Check-up topology and correct errors. Run topology tests until all errors are corrected.
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