You are here: Technology Digitization Hydrography digitization

Hydrography digitization

Rate this item
(4 votes)

Input of hydrographic data (swamp polygons, other polygonal and linear hydrographic objects, point objects and their characteristics) 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. Extraction of an image with swamp strokes.
  2. Recognition of swamp polygons and form correction.
  3. Extraction of an image for vectorizing of linear hydrographic objects.
  4. Automatic vectorizing and processing of linear hydrographic objects.
  5. Water edge recognition.
  6. Processing of rivers represented by double lines.
  7. Check-up and line form correction.
  8. Recognition of lake polygons.
  9. Assembling of main channels; separation of intermittent river segments.
  10. Line form optimization and line adjustment.
  11. Object ascription to their layers; input of common attributes.
  12. Topology check-up.
  13. Check-up of water edge positioning and input of their elevation values; generation of inscriptions.
  14. Input of water bodies' names; manual vectorizing of river characteristic and current direction symbols, rapids, etc.
  15. Final topology check-up.

Step 1. Extraction of an image with swamp strokes

The objective of the first step is image preparing for subsequent automatic vectorizing. It is possible to vectorize everything at once of course and to correct generated mistakes later but we know from practice that this approach is less effective than one with some time spending for preprocessing.

We want to extract strokes of swamps from other lines in the image, to mender short gaps in them, and to delete degenerate strokes. The Mask Filtering utility performs these tasks rather well. Applied settings (the "Swamp line extraction" strategy) are included into the program distribution kit. You need only select it and click the "Start" button.

It deserves mention that vectorizing of any material becomes significantly more effective when a corresponding image is prepared beforehand.

The source image
The image after processing
  1. Make a copy of the source image and call it "Swamps". Do not make alterations in the source image!
  2. Open the "Swamp" image for editing and apply the "Swamp line extraction" strategy of the Mask Filtering utility.

Step 2. Recognition of swamp polygons and form correction.

The prepared image serves as source material for automatic vectorizing at the second step with subsequent processing of raw vector data with the Autodetect Swamps utility. It is clear from its name that the utility recognizes swamps proceeding from vectorized individual strokes and form swamp polygons. Functioning of the utility depends on a rather big set of parameters.

A three-step Wizard helps to specify all parameters of the utility. The first page contains conditions of stroke selection for the operation, the second – rules and limitations of polygon forming, and the third – final data characteristics. Process parameters are rather easy to specify in spite of their big number – some of them may be "taken" directly from the screen and others – selected taking intermediate results into account. You may also return to the previous stage of the Wizard if necessary.

Remember that optimal parameters may differ even for adjacent map sheets. So, glance over the entire project quickly before you click "Save" to determine if some swamps remain unrecognized. If yes, try to change recognition parameters. Parameter selection is anyway a quicker operation than manual editing of incorrectly recognized objects.

Initial strokes
Swamp polygons after recognition
Polygons without use of the "Smooth resulting polygons…" and "Generate caps…" options
  1. Vectorize the "Swamps" image applying the Autotrace Lines utility.
  2. Form swamp polygons applying stroke recognition by the Autodetect Swamps utility.
  3. Review the project field systematically applying the Inspector tool and correct the shape of polygons with the Vector Eraser and Cumber Editor tools.
  4. Check the polygons applying the "Swamp polygons" strategy of the Topology Check-up utility and correct detected errors. Repeat the check until the number of errors is equal to 0.
  5. Select all swamp polygons with the Group Editor and ascribe them to the "SOILS AND LAVA NAPPES_pol" layer.
  6. Without object deselection, assign the _OBJ_TYPE_ attribute to all of them at once.
  7. Swamps of another type (if any), select separately and assign them the "Impassable swamps" attribute.

Step 3. Extraction of an image for vectorizing of linear hydrographic objects

Vectorizing of linear objects should be also preceded by extraction of a special image. Swamp strokes in the source image are unneeded at this step and so are numerous inscriptions. Even though these objects are detached from linear rivers and lakes, they will hamper effective use of data filtering and recognition utilities. It is recommended to delete all "rubbish" objects before use of these utilities. Some of "rubbish" objects can be deleted only manually but image processing before vectorizing helps to avoid forming of most of them.

Image before rasterization

 

... Image after rasterization and thinning
  1. Make a copy of the source image and call it "Hydrography".
  2. Delete swamp strokes from the image applying the Rasterize Vector Data utility, i.e. imprint swamp polygons into the image with the black color.
  3. Delete numeric characters from the image applying the Autodetect Topo-Symbols utility.
  4. Apply "Holes Filling" strategy of Mask Filtering with the "Slits 1*1" filter (there are many short gaps in lines in our case study images apparently caused by a scanning defect.).
  5. Run the Image Thinning command to receive "skeletons" of raster lines.
  6. Apply "Large Garbage Removal" strategy of Mask Filtering with the "Slits 4*4" filter to delete rubbish objects from the image.

Step 4. Automatic vectorizing and processing of linear hydrographic objects

The image is prepared, unneeded objects are deleted as far as possible, and line skeletons are extracted. It is possible to start vectorizing now. Resulting vector data will be processed with a series of utilities to minimize the share of manual editing. The utilities will snap line ends to the frame, restore dashed lines, delete artifacts (branches, strokes), and join line segments across long gaps.

Intermittent rivers before break-up joining
... and after joining
  1. Vectorize the image applying the Autotrace Lines utility.
  2. Snap line ends to the frame applying the Topology Correction utility.
  3. "Sew together" individual strokes in dashed lines representing intermittent rivers applying "short" break-up joining with the utility of the same name.
  4. Clean vector data with the Raw Line Filtering utility.
  5. "Sew together" long gaps in lines applying the Break-up Joining utility.

Step 5. Water edge recognition

Integrity restoration of river lines broken by water edge marks is one more operation, which can be performed automatically. The marks will be recognized as individual objects and attributed to the corresponding layer, whereas gaps in lines caused by these marks will be mended. There is a shortcoming as well – small rounded lakes may be mistakenly recognized as water edges. But these errors are easy to detect and correct. It is much easier to correct several falsely recognized lakes than to input several tens of water edge marks per a map sheet.

Water edges before recognition
... and after recognition
  1. Use the Autodetect Circles utility to free river lines from water edge marks.
  2. Check the results. The utility could recognize small lakes as the marks. These objects should be returned to the Auto_Hydrography layer. To return a lake, select the object with the Editor and run the "Convert" command in the context menu.
  3. For quick and convenient review of all water edge marks, select them all with the Group Editor and mark. Then use the "F" key for navigation between marked objects.
  4. Convert recognized circles into points with the Object Conversion utility.
  5. Select the points with the Group Editor and assign them the "Water edge" attribute.

Step 6. Processing of rivers represented by double lines

Double lines of constant width represent rivers from 5 to 30 m wide in topographic maps. According to the digital model requirements, such a river should be vectorized as a single line coinciding with the river center line. Automatic vectorizing generates double lines of course but there is a semiautomatic tool for center line forming in Easy Trace.

There are not so many rivers of this type in a map sheet as a rule - one or two most often. They may cross the entire sheet, but line length has no influence upon processing time.

River before processing
... and after processing
  1. Restore the less broken side of the river.
  2. Move the resulting vector line to the center of the corresponding raster line applying the Shift / Resize Polyline tool (hot key R).
  3. Delete the line representing another side of the river.

Step 7. Check-up and line form correction

Use of automatic utilities is over. It's time to correct remaining defects manually. Apply the Inspector tool to ensure systematic review of the project without omissions. This is the most laborious part of the work but it seldom takes more than an hour per a map sheet if you master editing tools well.

Double-click line joining accelerates the operation a lot. To join line ends together, place the cursor in the gap between them or nearby so that the line ends you want to join were the nearest to the cursor, and click the mouse left button twice. This feature is provided for most editing tools (Vector Eraser, Cumber Editor, Editor, Topology Editor) and you needn't waste time for tool change.

Lines after automatic vectorizing
... and after several seconds of editing with the Vector Eraser
  1. We recommend you to switch the Polyline Ends view mode on before data review. It will help to notice line discrepancies and small rubbish objects. Apply the Inspector tool to review the project field systematically.
  2. Apply double-click line joining to delete gaps.
  3. Use the Cumber Editor for line form correction.
  4. Use the Vector Erase tool for object deletion, cut, and correction.
  5. Input unrecognized symbols of water edges with the Point tool to the "HYDROGRAPHY(RELIEF)_pnt" layer.
  6. Mend big gaps in lines applying the Curvilinear Tracer.
  7. Correct sites of river junction applying the Topology Editor.

Step 8. Recognition of swamp polygons

Data review and manual correction ensures elimination of all mistakes in vector data and enables applying of one more automatic utility. It assembles lake polygons represented by line fragments after automatic vectorizing. The utility also recognizes isolated lakes already represented by polygons.

Lakes before recognition
... and after recognition
  1. Extract polygons of lakes and attribute them to the "HYDROGRAPHY_pol" layer applying the Autodetect Lakes utility.

Step 9.Assembling of main channels; separation of intermitted river segments

Any adjoint segment breaks the river line into segments. On the other hand, any river always has the main channel, which should be represented by an integral line. Some adjoining segments may be recognized together with the main channel forming by the Raw Line Filtering utility. Unobvious sites should be processed manually.

Manual processing is also inevitable for dashed lines representing intermittent parts of the river. The simplest way out is attribution of intermittent rivers to another layer, as it takes only one stroke of the corresponding hot key.

Lines before processing
... and after processing
  1. Apply the "T-Joints" option of the Raw Line Filtering Utility to join and correct river lines at junction points.
  2. Assemble the main river channels applying manual joining where filtering has failed. Split falsely joined segments with the Editor (the "W" hot key) or use the Vector Eraser in the blue (cutting) mode.
  3. Make sure that only two vector layers ("Auto_Hydrography" and "Intermittent") are visible. Select intermittent rivers and river parts and ascribe them to the "Intermittent" layer. Use the "Split polyline" mode of the Editor (the "W" hot key) to cut off intermittent segments of rivers. The cut off segment usually becomes selected automatically as the shortest ones and it remains only to assign it to another layer (the "S" hot key).
  4. Assemble parts of polygonal rivers. Do not snap river line ends to the frame to avoid distortions at line form optimization.

Step 10. Line form optimization and line adjustment

These were dense lines generated at automatic vectorizing that undergone processing so far as they are easier to alter and correct. The Line Form Optimization utility deletes superfluous vertices when this work is over.

Different optimization parameters are required for different object groups. Parameter sets (strategies) are already created and it remains only to select them from the list. Optimal parameter values depend on line flow and should provide correspondence of vector and raster lines.

Lines before optimization
... and after optimization
  1. Optimize line form.
  2. Optimize line form for lakes. Operation parameters are milder than ones for river line optimization otherwise lake shape may suffer.
  3. Optimize line form for intermittent rivers. There are prominent bends and kicks in these lines after assembling out of strokes and thus these lines require most stringent optimization parameters.
  4. Line form optimization has caused form change and mismatching of objects. Run the Topology Correction utility to adjust them.

Step 11. Object ascription to their layers. Input of common attributes

It is inconvenient to use layers of Panorama GIS for some reasons while editing and Easy Trace ascribes vectorized objects to intermediate layers ("Auto_Hydrography", "Intermittent"). When this work is over, the object should be attributed to their final layers. Layers taken from Panorama already have attribute tables and domains as well as preset thematic displaying of objects. Besides, all topological check-up strategies are also prepared for these layers as they should check not individual map sheets only but also the entire coverage.

Before ascription
... after ascription
  1. Select all lake polygons with the Group Editor, transfer them to the "HYDROGRAPHY_pol" layer and ascribe them the "Lakes permanent" attribute.
  2. Snap polygonal rivers to the frame (the "D" hot key), transfer them to the "HYDROGRAPHY_pol" layer and ascribe them the "Rivers permanent" attribute.
  3. Transfer permanent linear rivers to the " HYDROGRAPHY _lin" layer and ascribe them the "Rivers permanent (less than 5)" attribute.
  4. Transfer intermittent linear rivers to the " HYDROGRAPHY _lin" layer and ascribe them the "Rivers intermittent (less than 5)" attribute.
  5. Transfer rivers represented by double lines to the "HYDROGRAPHY _lin" layer and ascribe them the "Rivers permanent (from 5 to 30)" attribute.

Step 12. Topology check-up

Topology check-up consists in execution of special tests. Every test includes one or several layers to be checked and types of mistakes to be detected. At that, tests may be saved as strategies that in turn may be combined into groups to be run together. This is extremely convenient. For example, there is no point in relief check-up when you process hydrographic objects. On the other hand, a total check before project delivery is desirable to guarantee absolute data correctness.

Another advantage of this approach is portability of check-up settings. They may be adjusted in one project and imported into another (with similar structure of vector layers!) or inherited automatically if the first project serves as prototype at new project creation.

  1. Use the Topology Check-up utility to verify topological correctness of vector data. Run the tests until the number of errors is equal to 0.

Step 13.Check-up of water edge positioning and input of their elevation values; generation of inscriptions

Manual editing of all water edge symbols is required to provide their consistency with hydrographic objects and attribute (elevation) input. As height mark of every water edge should be placed at the base of inscription (which is also needed in the digital form), use the function of automatic inscription generation.

The generated inscription will represent the elevation value you have input applying the settings you have specified (layer, inscription type, text height). Put every vector inscription above the corresponding raster one for correct positioning and easy search for misprints at elevation value input

.

  1. Set up generation of text inscriptions and attribute assigning to them.
  2. Select all water edge marks with the Group Editor and mark them.
  3. Use navigation between marked objects (the "F" hot key). Adjust water edges with hydrographic objects. Activate the "Generate inscriptions" and "Edit inscriptions" options in the Object Attributes dialog box. The inscription generated after elevation value specifying will "hang" by the cursor. Place it in the correct position.

Step 14. Input of water bodies' names; manual vectorizing of river characteristic and current direction symbols, rapids, etc.

River characteristics, current direction, rapids and other symbols should be vectorized manually at the final stage of vectorizing. It also comprises attributive data input for hydrographic objects (name, navigability, refined type).

Besides, it's time to cut islands out of polygonal rivers.

  1. Switch on the image with black objects to make current direction indicating lines visible. Switch on the "Thematic Displaying" view mode to make data representation more obvious.
  2. Review the project field systematically applying the Inspector tool. Input names of water bodies. Separate polygonal water bodies at name change applying the "Autoclose" mode of the Curvilinear Tracer.
  3. Vectorize current direction indicators, river characteristics, rapids, and other objects with the Curvilinear Tracer in the manual mode.
  4. Cut islands out of polygonal water objects, ascribe them to the "HYDROGRAPHY(RELIEF)_pol" layer and assign the corresponding attribute.

Step 15. Final topology check-up

Topological correctness could be damaged at attribute input and object transfer to other layers. Besides, new objects are added. That's why final topology check-up is necessary before you consider hydrography vectorizing accomplished.

  1. Check topological correctness applying the Topology Check-up utility. Run the tests until the number of errors is equal to 0. It is better to make settings of check-up strategies once in the project-prototype. They will be inherited by all new prototype-based projects.
More in this category: « Prev Next »

Add comment


Security code
Refresh