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Slant Range vs Horizontal Range and Calculating Waypoints


Slant Range Distortion
The ranges measured by a basic side scan sonar record are slant ranges from the sonar to each point recorded (see “Sonar Geometry” section). Unless an automatic correction is applied to the data, these will also be the ranges when the data is displayed. Inherent in a slant range display are two principal distortion factors which should always be kept in mind and will often need to be corrected by some means.

The first is, the distances to features on the record are not horizontal distances to either side of the sonar. That is, we cannot look at a record and directly measure the offset of a feature from the ship’s track. This must be found by calculation, and will be treated separately under the heading “Horizontal Offset Calculation”.

Secondly, targets are compressed in the across-track direction. The difference in slant ranges to the leading and trailing edges of a target will always be less than the actual extent of the target. The degree of compression varies with the horizontal distance to the target. The closer the target to the sonar, the more severe will be the compression. At greater distances the slant ranges are closer to the true horizontal ranges and the compression effect is less severe. Actual target extent in the across-track direction can be found by the same calculations used for true horizontal offsets. The effect is shown schematically in the illustration below.

The records below show a portion of sonar image before and after correction for slant range and speed distortions. Note that the feature closer to the sonar is changed more by the correction process than the features further away.

Horizontal Offset Calculation
Many side scan sonar applications require that we be able to accurately pinpoint the locations of targets seen on the records. A basic side scan sonar system (that is, one not including an automatic correction opt ion) displays slant ranges from the sonar to features on the bottom. For accurate positioning we must determine the horizontal distance from the sonar to these targets. As discussed in the “Slant Range Distortion” section, the distorting effect is greatest at the shorter ranges from the sonar.

Horizontal offset can be found by calculation using the Pythagorean theorem. Referring to the above schematic, In the following example, the towfish height is one leg of the right triangle, measured as 15 meters. The measured slant range is the hypotenuse,measured as 27 meters, and the horizontal offset or range is the other leg (side scan sonar always assumes a flat bottom). The horizontal range is then:


I hope this helps,


Ok, here is a diagram by Tom Vickers from Humminbird (with a few enhancements from me) that I think will help aid in and maybe stir a little discussion.  There are some very interesting objects shown and how they will appear on the Side Imaging view using 455khz. 

I think whats also interesting is that this diagram was drawn long before Down Imaging was even thought of!  Notice how the object directly under the boat is displayed as beeing cut in half.



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