Source raster data

Step 1.Extraction of relief lines into a separate black-and-white raster.
Step 2.Grid removal: raster to vector conversion
Step 3.Grid removal: imprinting of vector lines into the raster
Step 4.Filtration of the relief raster
Step 5.Automatic raster to vector conversion
Step 6.Joining of ruptured polylines
Step 7.Removal of vectorized inscriptions
Step 8.Line shape improvement and correction of raster to vector conversion errors
Step 9.Assignment of height values to isolines
Step 10.Export of tracing results into the target GIS

Appendix: Step-by-step instruction for an independent fulfillment of the described process.

Load raster file COLOR.TIF (ZIP-archive)

Automatic digitalization workflow is considered for an example of a topographical map on the 1:24000 scale being stored in a foreign fund of digital data.

To "reduce all the data to a common denominator", all rasters of the fund were normalized, i.e. they have the same 18-color palette. Raster resolution is 254 dpi.

Due to such a radical palette reduction, most objects in the raster are represented by sets of the same colors with predominance of one or another color.

Trying to extract relief isolines from the raster, you capture inevitably some other objects - inscriptions, grid lines, etc.

A similar situation can result from low printing quality of source materials, interpenetration of colors, tint forming by color dilution with white dots, etc. Till recently, such rasters could be processed semi-automatically only, and the manual labor share was great.

Load project AUTO.JET (ZIP-archive)

Initial state: an Easy Trace project is created, and the color raster COLOR.TIF is added to it.

Applying the Binarization tool , extract relief lines (select the dark-brown tint only, or you will capture a great number of parasitic objects).

Nevertheless, fragments of grid lines and inscriptions are present in the resultant black-and-white raster. This "rubbish" would cause a great editing effort after auto tracing.

Add the resultant raster to the project AUTO (the Project - Add Raster command, without transformation, as the raster COLOR.TIF).

To delete grid remains from the raster, we shall apply the Rasterization utility.

Trace the grid on the color raster manually applying the Broken-line tracer at line width 4 pixels and put these vector lines to the layer "Grid". The lines are wider a little than the raster grid lines and therefore will cover them for sure.

Sometimes it is better to execute this step and the next one on the original color raster (before separation of the Relief color layer, especially if you are going to separate more than one subject layer (e.g., relief, rivers, roads, etc.).

Now let us select black color for rasterization and convert vector grid lines into raster ones.

Manual erasure of grid remains with the Brush tool takes much more time.

More details about object removal...

How to estimate the results of raster filtering in Easy Trace...

See below results of the "Thin isolines" strategy use.

Filtering of black-and-white rasters allows you to solve several problems simultaneously, i.e., to improve line shape, to fill short gaps, to delete "rubbish" and parasitic"pasting" of neighboring lines.

This operation consists of consecutive use of specialized filters (a "strategy" in our terms). Practise a little, and you will be able to form your own strategies.

The following automatic tracing parameters are preferable for thin (1-3-pixel width) lines:

You should specify two parameters only to join ruptured lines:

• the layer of the lines to be repaired;
• maximal gap to be "sewed up".

Draw attention please to polyline shape recovery in the gaps resulted from river bends.

Let us make use of small length of the polylines - traced inscriptions that allows one to select and delete them with the Group Editor .

• Select segments shorter than 50 pixels.
• Check up the selected line set (left mouse button - select the line, right button - deselect the line).
• Delete the selected lines.
  

To accelerate the process of line merging, it may be necessary to "snip off" curved isoline ends with scissors taken in the Editor's submenu.

At that, if you hold the <Ctrl> button pressed while pointing the line, its shortest segment will be not only cut off but also deleted automatically.

The new "Shear" tool may be useful sometimes. It allows you to snip off and delete all line fragments within (or outside of) an arbitrary polygon. You will find the tool in the Group Editor's submenu.

A. The Group Editor tool. B. Additional ("Right-click") menu of the Group Editor tool; The Shear tool. C. Cutting polygon

After "cleaning" of hard areas you may run the utility of line merging again, applying a greater value of maximal gap to be "sewed up" (30 - 50 px).

Revealed errors:

• Self-intersection
• "Cross" intersection
• "Common vertex" intersection
• Dangling nodes
• Pseudo-nodes

To improve polyline shape, we shall use the following parameters:

Use the utilities of topology verification to detect isoline crossings and gaps in the lines.

Correct the revealed errors with the help of the Editor's submenu commands .

We shall use the Z-tool to form valid relief lines.

Load final ArcView project (ZIP-archive)

Export to your GIS (ArcView in our example) is the last operation that remains to do. You may also select other output formats: DXF, MIF, GEN, DGN, CSV, ASC, and TOP. Pay your attention please that a world-file containing raster registration data is created at export.