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SI Enhance Contour Mode Question
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From Robert Gecy at sideimagingsoft.com ...
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 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.
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So if we take your distance and depth scenario ... It figures close to this ...
Horizontal Range at your scenario is approximately 96.82' left and right ...
Contour Mode does not change this because "Contour Mode Off" (water column showing) basically compresses the bottom detail closer to the water column ...in order to make room in the SI image for the water column ...
Notice in the Contour Mode depiction above that the bottom detail at the very far left and right of the display are exactly the same data detail size for "Contour Mode Off" and "Contour Mode On" ... It's the bottom detail closer to the water column that is "compressed" to make room in the SI image for the water column data with "Contour Mode Off"...
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Display size has no bearing on the Slant Range/Horizontal Range calculations ...(The total "underwater acreage" encompassed by the "SI Range" setting is processed to fit into the display size that is attached to that specific processor) ...
Rickie
Romans 5:8
