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Relief digitization

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Input of relief data (vectorizing of contours, symbols of stones, relief elements, and elevation data input) takes 2 - 2.5 hours per one sheet of a topographic 1: 50000 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 relief data (vectorizing of contours, symbols of stones, relief elements, and elevation data input) takes 2 - 2.5 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 elevation marks and assignment of Z-values.
  2. Image refinement.
  3. Image preparing for recognition of stones.
  4. Automatic outlining of stones and processing of resulting vector contours.
  5. Manual processing of stone contours and relief elements.
  6. Preparing of image with relief contours.
  7. Automatic vectorizing of relief contours and processing of resulting vector lines.
  8. Check-up and defect correction.
  9. Line form optimization.
  10. Type assignment to contours.
  11. Recognition of stone clusters.
  12. Topology check-up.
  13. Assignment of elevation values.
  14. River orienting.
  15. Elevation check-up.
  16. Input of slope-indicating hachures.

Step 1. Recognition of elevation marks and assignment of Z-values

It is possible to tell points of elevation marks from other objects automatically on maps at the scale of 1 : 50 000. Recognition of special elevation symbols like geodetic stations is even simpler.

The objective of the first step is vectorizing of points – elevation marks. The Autodetect Topo-Symbols utility recognizes most of the points and subsequent review of the material enables you to delete false object and to add missing ones manually.

A fragment of source material
Recognized elevation points are marked by the utility
  1. Recognize elevation marks in the image applying the Autodetect Topo-Symbols utility.
  2. Review the project field systematically applying the Inspector tool to delete objects recognized by mistake and to add omitted ones.
  3. Review the objects and assign Z-values to them. Select and mark all points at once with the Group Editor, and then apply quick navigation between marked objects. Switch on corresponding options for automatic positioning of the working window and automatic opening of the Object Attributes dialog box.

Step 2. Image refinement

Automatic vectorizing always starts with image preparing, even if the images are already converted into black-and-white ones. It is necessary at least to delete small "rubbish objects", to fill caverns in lines and to repair short gaps.

There are scanning defects in long lines in the form of vertical gaps. They should be corrected by Mask Filtering with the "Fill caverns" strategy, mask 1*1.

Image before mask filtering
.. and after filtering
  1. Open the image and apply the Mask Filtering utility with the "Fill caverns" strategy, mask 1*1.

Step 3. Image preparing for recognition of stones

Separate processing of different elements is the base of many methods we successfully apply and recommend to our users. Divide and conquer, as the saying goes!

Vectorizing of relief contour lines is problematic at sites of there crossings with symbols of stones. Besides, these symbols should also become vector objects. This is a good example how one can kill two birds with one stone. Symbols of stones are extracted to a separate raster layer and vectorized automatically with manual deletion of false objects. After that, we "inprint" vectorized stones into the prepared (thinned) image of relief contour lines and thus delete most bridges and other artifacts.  The same approach is applicable to relief elements.

This unloading makes the image suitable for automatic vectorizing of relief contours. Resulting lines require only minimal manual editing after being processed by a series of automatic utilities.

Let us consider the process stepwise. Extraction of stone symbols first of all.

Before extraction of stone representing spots
After extraction of the spots
  1. Make a copy of the source image and call it "Stones".
  2. Оpen the image and extract spots of stones applying the Spot Separation utility.

Step 4. Automatic outlining of stones and processing of resulting vector contours

The image is prepared, irrelevant objects are deleted, and stone-representing spots are extracted. Vector contours of stones will be used for spot deletion from the image and for generation of point objects representing individual stones and stone clusters.

Before outlining
... and after outlining.
  1. Process the image of stone spots with the Outline Contours utility.
  2. Smooth resulting contours.

Step 5. Manual processing of stone contours and relief elements

The objective of this step is deletion of "rubbish" objects, adding of omitted stones, and vectorizing of relief elements.

Special elements of relief actually defy automatic vectorizing and cause many defects at vectorizing of contour lines. It is possible to simplify further work significantly by beforehand vectorizing (even if manual) of the elements and their erasing from the image.

Relief elements before vectorizing
... and after vectorizing
  1. Review the project field systematically applying the Inspector tool.
  2. Delete all irrelevant objects (i.e. ones different from contours of stones) with the Eraser tool. Add omitted stones with the Point tool. Input points at the spot centers and attribute them to the layer of stone contours (Auto_stones).
  3. Vectorize linear and polygonal relief elements applying the Curvilinear Tracer in the "Spline" mode. Attribute linear elements to the "LAND RELIEF_lin" layer, and polygonal elements to the "LAND RELIEF_pol" layer.
  4. Input attributes of the relief elements.

Step 6. Preparing of image with relief contours

It's time for the "Divide and conquer" motto. We can now delete vectorized symbols of stones and relief elements from the image and thus make automatic vectorizing of relief contours more effective and manual editing less laborious.

It is reasonable to apply Mask Filtering after erasing of stones and relief elements from the image to repair small gaps in lines.

Image before stone erasing
> ... and after erasing
  1. Make a copy of the source image and call it "Relief".
  2. Apply "Thick isolines" strategy of Mask Filtering. The "Soft line separation" filter should be on.
  3. Extract centerlines with the Image Thinning utility.
  4. Delete remnants of stones, relief elements, and frame from the image.
  5. Sew together lines across gaps applying the "Thin isolines" strategy of Mask Filtering.
  6. Delete "rubbish" from the image applying the "Large garbage removal" strategy of Mask Filtering.

Step 7. Automatic vectorizing of relief contours and processing of resultant vector lines

The image is ready – irrelevant objects are deleted and lines are thinned. Apply the Autotrace Lines utility for automatic vectorizing.

Vectorized contour lines should be processed with a series of utilities that improve vector data and minimize manual editing. These utilities snap lines to the frame, delete vector artifacts (branches, strokes, etc.) and repair close up gaps.

Lines before automatic vectorizing
Lines after automatic processing
  1. Vectorize the image applying the Autotrace Lines utility.
  2. Delete small defects with the Raw Line Filtering utility. Input operation parameters by defect pointing on the screen. Repeat filtering until the number of all defects is equal to 0.
  3. Snap line ends to the frame applying the Topology Correction utility. It will help to avoid incorrect line joining across gaps.
  4. Sew up short obvious gaps in lines applying the Breakup Joining utility.
  5. Delete remaining defects with the Raw Line Filtering utility. Specify operation parameters so that defect correction would not cause deletion of line segments. Repeat filtering until the number of all defects is equal to 0.
  6. Sew up long gaps in lines applying the Breakup Joining utility.

Step 8. Check-up and defect correction

Manual editing is necessary for line discrepancy correction and vectorizing of contours at complicated sites automatic utilities can not manage with. Switch on the "Polyline ends" view mode for the convenience of editing.

A double click in the gap between lines is the quickest way to sew them together. All primary editing tools support this function and you may waste no time for tool change.

The Camber Editor tool is convenient for line form improvement. Also use the Vector Eraser to delete, cut, and correct contours.

  1. Run the "Nodes of degree more or equal 3" test of the Topology Check-up utility to find line agglutinations and crossings.
  2. Navigate from one error mark to another (hot key F) and correct the errors.
  3. Correct other defects of contour lines. Review the project field systematically applying the Inspector tool.
  4. Draw isolines along linear cliffs applying the Copy mode of the Curvilinear Tracer. Isolines running into the cliff should be drawn along the edge.
  5. Use the Topology Editor to snap isolines running into relief elements to these elements. If the distance is long, point the isolines with the mouse right button to capture it and then draw the missing part. Draw manually isolines running through relief elements (use copying).
  6. Sew polylines together at pseudo-nodes applying the Breakup Joining utility.

Step 9. Line form optimization

Vector lines being edited remained dense until now, just as they had been generated at automatic vectorizing. Dense lines are more convenient for shape correction with the Eraser tool as vertex adding is a seldom task. Superfluous vertices should be deleted with the Line Form Optimization utility after line form improvement is generally over.

Deletion of superfluous vertices ensures quick functioning of utilities for topology check-up and correction, automatic input and check-up of Z-values and others.

  1. Smooth the lines and delete superfluous vertices with the Line Form Optimization utility.
  2. Unite lines at pseudo-nodes. It was impossible to join some segments before optimization as resulting lines had more than 8192 vertices. Optimization has decreased the number of vertices and the segments will be joined.

Step 10. Type assignment to contours

Manual assignment of types to isolines is an important step at relief vectorizing. Three types are mostly in use (Intermediate, Supplementary, and Index).

Elevation increment and representation parameters differ for contour lines of different types. The first circumstance enables relief structure reading by automatic utilities and the second helps the operator to avoid mistakes at result check-up.

We know from our own experience that raster lines lying under bright vector objects are difficult to see and one has to zoom in the working window constantly to determine their types. The way out is to damp vector lines, i.e. to make them almost black (yet not black!). It is also possible to switch on the color image during the operation if you don't want to take trouble over line colors. Not so convenient, but easy.

 

Relief contours before type assignment
Relief contours after type assignment
  1. Specify an almost black color in thematic view for isolines without the attribute (Other Value).
  2. Review the project field systematically applying the Inspector tool. Use the Group Editor for polyline selection and the F2 key for opening of table of attributes. Assign the attribute (contour type) and stroke Esc to reset the current selection.
  3. Assign the attribute to Index and Supplementary contours, then select all isolines without the attribute at once with the Group Editor and make them of Intermediate type.

Step 11. Recognition of stone clusters

The "Stone cluster" topo-symbol actually consists of three "Individual stone" symbols placed nearby. It proved to be rather simple to distinguish between these two objects automatically.

The Detection of Stone Clusters utility analyzes if input point objects belong to triplets. It interprets detected triples as stone clusters and remaining points as individual stones. The later are being converted into short strokes (to become objects of the "vector" type in Panorama GIS). Then the utility puts the objects to the corresponding layer and assigns them the specified attribute.

As the Detection of Stone Clusters utility needs points as input data and automatic vectorizing of stone symbols produces polygons, it is necessary to convert the polygons into points with the Object Conversion utility.

Stone contours before conversion
.. and after recognition and attribute input
  1. Convert stone contours into points.
  2. Tell stone clusters from individual stones in the resulting set of points applying the Detection of Stone Clusters utility.

Step 12. Topology check-up

Similar to processing of other subject layers, it is necessary to check-up topological correctness of the digital model being created at final stages of relief vectorizing.

The "Relief" layer is usually the simplest in this sense though, and ready test sets (strategies) enable you to check topology and correct mistakes rather quickly.

  1. Run topology check-up tests and correct detected errors. Repeat the operation until all topological mistakes are corrected.

Step 13. Assignment of elevation values

Input of Z (elevation) data comprises an automatic and a manual stage. Z-parameters should be specified first of all (if it has not been done yet). It is necessary to specify layers of contour lines and there representation depending on the contour type. You may select different width and line type for Intermediate, Index, and Supplementary contours and adjust line colors depending on Z values.

The Z-assignment utility automatically inputs elevation values of 50 - 70% of contours. Its effectiveness mainly depends on the nature of material and scattering of elevation marks. Correctness of relief topological model in the image is also of great importance. The utility ignores a whole bunch of contours sometimes just because of one mistake made by map compilers.

The semiautomatic Z-tool enables elevation assignment to remaining contour lines.

Contour lines before Z assignment
Contour lines after Z liniment
  1. Turn values of the "True altitude" attribute of elevation marks into Z values applying the Transfer Z to/from Database utility.
  2. Specify Z parameters.
  3. Start elevation input with the Z tool (Alt+Z to open). Use the "Pointing" and "Crossing" modes of the tool functioning. Press "F" to move to the next contour without a Z-value.

Step 14. River orienting

According to the digital model requirements and from general considerations, direction of river lines in the river network model should coincide with current direction.

It is easy to determine the set of current automatically for long rivers when a digital model of relief is disposable. Besides, this operation serves as a crosscheck and helps to correct errors in both relief and river network models. Short river segments are more difficult to process. Nevertheless, the utility does its best and marks the segments for subsequent manual correction if fails.

  1. Correct direction of river lines applying the Orient Rivers utility. Correct detected mistakes.

Step 15. Elevation check-up

The next step consists in relief model check for consistency of Z-values assigned to contour lines and ones of elevation marks. We recommend including of crosscheck with rivers in this test even if it resembles the functioning of the Orient Rivers utility.

Among other things, the Z Check-up utility can direct contour lines as it is specified in the digital model requirements.

  1. Check correctness of assigned elevation values applying the Z Check-up utility. Correct detected mistakes. Contour breakups in bottlenecks may cause detection of false errors.

Step 16. Input of slope-indicating hachures

Slope-indicating hachure (bergstrich) input concludes relief processing. It would take much time in the manual mode but most slop indicators on closed contours will be generated automatically. Besides, the mode of automatic block snap and directing enables one-click input of remaining bergstrichs.

  1. Input stroke-indicating hachures applying the Generate Hachures utility.
  2. Input remaining hachures manually applying the Block tool. Specify any bergstrich with the mouse right button to select the corresponding block. Select "By polyline" option with 90 deg. value for the "Angle" parameter. Review the project field systematically applying the Inspector tool.
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