Both the comparable vectorizers provide means for subject layer extracting from color rasters: Classification (R2V) and the Binarization tool (Easy Trace).

As preliminary viewing of results (Preview) is IMPOSSIBLE, one should rely on intuition only at layer extracting. After an unsuccessful attempt, you have to reload the raster to try again, as the Undo operation is ALSO UNPROVIDED.

We have managed to extract the isoline layer after numerous attempts (fig. 2), but failed to delete completely all black objects (the grid and inscriptions) from it. Besides, there were a lot of gaps in isolines.

Fig.2. The isoline subject layer extracted to a black-and-white raster by means of R2V.

The Easy Trace vectorizer has an integrated Rainbow package for subject color separation. It's a set of specialized tools allowing preliminary viewing of results. It took us less than one minute to extract the isoline subject layer. At that, there were neither black objects in the extracted layer (fig. 3) actually, nor long gaps in isolines. Remnants of the grid and inscriptions are formed by "boundary" brown pixels appeared at raster normalizing before its insertion to the fund.

Fig. 3. The same subject layer extracted to a black-and-white raster by means of Easy Trace.

Objects that don't belong to the current subject layer should be deleted from it at this stage. These are grid and inscription remnants in our case.

The R2V vectorizer provides the Image Pixel Tool for this purpose. It enables manual erasing of unnecessary pixels and drawing of missing ones. Actually, this tool is USELESS at real raster processing, as its use takes more time than editing after automatic tracing.

Besides the Brush tool similar to the Image Pixel Tool, Easy Trace provides vector data subtraction from raster. To delete grid remains, you may:

1. Digitize the grid on the color raster applying the Broken line tracer.
2. Apply the Rasterization utility to the obtained vector grid.

It is reasonable to make vector lines wider a little than the raster grid lines for complete deletion of the grid from your raster. Gaps left by the grid in isolines will be partly filled at raster filtering, and partly - at merging of ruptured polylines.

The Rasterization utility is also suitable if you want to delete remnants of other objects (rivers, roads). Its use is particularly reasonable if the objects making obstacles for automatic tracing should be also traced in your project. For example, after tracing of a river network, you may copy it to the relief layer, increase line width, and then "subtract" it from the isoline raster.

To delete accidental noise, R2V has only the Despeckle filter without adjustable parameters. Repeated applying of the filter is recommended in the User Guide to increase "purification efficiency". It causes extinction of polylines 1-2 px wide (fig. 4).

Fig. 4. Extinction of thin lines in the raster at double use of the Despeckle filter.

Filtering algorithm in Easy Trace not only fulfills noise deletion at thin line preserving, but also partly restores ruptured lines and deletes parasitic"bridges" (fig. 5).The user may create different sets of filters optimal for particular rasters.

Filling of gaps in isolines "Rubbish" removal Improvement of the line shape Removal of parasitic"bridges" Fig. 5.

Fragments of black-and-white subject layers obtained with the help of R2V (fig. 6) and Easy Trace (fig. 7) are given below. One may see comparing the figures that most information is lost in the raster got in R2V.

Applying Easy Trace integrated tools only, we have managed rather quickly to obtain a raster suitable for automatic tracing.

Fig. 7. Absence of alien objects and high line quality makes the rasters
obtained in Easy Trace suitable for automatic tracing.

Automatic raster to vector conversion parameters of R2V allow the user to choose only a tracing mode - center of the line (Centerline) or contouring (Boundary), and a source data type - Map or Drawing. Lock of primary optimization at tracing gives rise to superfluous line segments and vertices.

Automaticraster to vector conversion tool in Easy Trace allows the user to specify predominant line type before tracing as well as auto-digitalization parameters - average line width in the raster, maximal size of spots and caves (holes) to be ignored, and the length of short (noise) vector segments to be deleted.

Polylines obtained by automatic raster to vector conversion are gaped on the spots of deleted grid lines and local defects of the raster. Both vectorizers have a tool for sewing of torn line ends together.

The Snap Line command is provided in R2V for sewing of torn line ends together. This command connects ends of polylines within a user-specified snapping distance. The best result achieved by repeated execution of the Snap Line command at consecutive increase of snapping radius is shown in figure 8. As is obvious, by far not lines are sewed together.

Most of the joints are incorrect. Besides, polyline ends on the raster edges are joined in pairs (upper part of the figure), and it is inadmissible for isolines.

The program ignores line direction at joining. There are no extrapolated vertices in joining segments that would provide slick joints considering line curvature.

Fig. 8. Joining of ruptured polylines in R2V. Some of evident joints were never done.
One can not consider the result to be satisfactory.

Easy Trace considers not only user-specified snapping radius, but also line direction: the joining segment extrapolates line shape basing on curvature of extremities of the lines being joined (fig. 9). It does not sew lines together on the edge of the project field. Actually, it joins 100 % of polylines.

Fig. 9. Joining of ruptured polylines in Easy Trace.

Both packages can delete short polyline segments caused by automatic digitalization of raster noise.

R2V deletes lines falling into a user-specified range of length or vertex number. It does not allow the user to deselect or add some objects to be deleted.

Some short unlinked segments will be deleted therefore, and some rubbish remains.

One of the main Easy Trace tools for vector editing - the Group Editor - enables you to select a set of objects to be deleted and afterwards to specify its membership more exactly. You may select polylines according to vertex number, and then add and delete objects from the set of selected ones. At that you may change the viewing scale and scroll the image.

Thanks to this feature, error number is minimal.

Resultant polylines have superfluous vertices that may be deleted without polyline shape distortion.

R2V has two commands for line shape improvement - Smooth Lines and B-Spline smooth. The best results we have achieved in the package are shown in figure 10. One can see that the Smooth Lines optimization distorts line shape (fig. 10, center), and the B-Spline smooth command forms numerous additional vertices although preserves line shape (fig. 10, right; superfluous vertices form red swells of the lines).

Fig. 10. Line shape improvement in R2V. One can see that the Smooth Lines filter (center) deletes superfluous vertices but distorts line shape. The B-Spline Smooth filter (on the right) preserves line shape but generates a great number of superfluous vertices.

When in Easy Trace, you may specify line type and approximation accuracy at line shape optimization. The procedure excellently deletes "unwanted" vertices at line shape preservation within any line part. Simultaneous smoothing is available controlled by adjustable parameters for different line types.

Line shape improvement is shown in figure 11. The processed polyline has minimal vertex number and well repeats the shape of the corresponding raster line.

Fig. 11. Line shape improvement in Easy Trace. The "happy medium" between vertex number and accuracy is found - the traced line coincides with its raster original.

Abilities of R2V in topology error detection come to nothing more than self-intersection of closed contours. Isolines going beyond the bounds of the map sheet are not closed, and therefore their intersections remain undiscovered.

Easy Trace automatically reveals and marks 10 types of topology errors. Unlike R2V, error marks are placed directly on the spots of the errors and indicate error types. Specialized means of vector editing allow automatic positioning of the working area at the spot of an error.

Topology verification criteria may be combined into verification strategies adjusted for specific object types and subject layers. These strategies are being stored in your project and inherited at new project creating.

The errors detected at the previous stage may be corrected by integrated topology editing tools in both packages.

R2V provides automatic correction of the errors, but we could never achieve acceptable results.

Easy Trace has a specialized tool allowing the operator to navigate quickly from one topology error to another without wasting time for their search all over the project field (fig. 12).

Fig. 12.

Impossible.

When in Easy Trace, you may assign Z-values to polylines both manually - applying the Editor - and semi-automatically. To do it, take the Z-tool, specify the step of Z value change and:

1. ... assign Z value to any polyline ...
2. ... click the line and then cross some next ones.
3. Repeat step 2 until Z values are assigned to all polylines.

You have only to control increase or decrease of Z values and don't miss the line that has gray (initial) color.

The program keeps tracks of intersections of "ridges" and "depressions" automatically. It constantly checks up correspondence of heights to previously assigned Z-values. Color Z-indication and Z-marks at the points where you have crossed the lines help you to control the process (fig. 13).

Fig. 13.

The afore-stated compare proves that the prevalent R2V vectorizer unfit to trace real isoline rasters. It has no necessary tools for isoline layer extracting from the initial raster and "cleaning" of the resultant image. Raster preparation for tracing takes a lot of manual labor or use of additional software is expected.

Abilities of R2V in traced data processing are also insufficient - joining of polyline ends and search of topology errors don't take specificity of isoline rasters into account. Automatic input of height marks is impossible.

On the contrary, Easy Trace has all the necessary tools for raster preparation, raster to vector conversion and vector import into your target GIS. Most labor-consuming operations (raster preparation, raster to vector conversion, joining of line ends, topology verification and height assignment to isolines) are computer-assisted and don't take operator's working time. At that, Easy Trace price is more than two times lower than one of R2V.