We have two Map Projects that we combine in a User Map, then Export as ESRI Shapefiles. The E2 Return Strength on one project is 0 to 209, on the other 0 to 102. In the User map, the Hardness Group has 12 elements from (-5.6 to 5.7) up to (53 to 65). Our intent is to be able to compare Hardness uniformly across the Shapefiles (i.e. "hard" in the area corresponding to one project is same reflectivity as "hard" in the area of the other project.) Will the Shapefiles allow us to make this comparison?

Calibrating "hardness" values between separate sonar logs is a challenge on consumer units with no real way of performing proper calibration (e.g. recording hardness values of a known object at a fixed depth). If possible, on separate runs you should try to record some data coincident with data from previous runs so that you can attempt to calibrate the logs using the offset and scale functions. E2 in particular is also sensitive to current weather conditions as this effects the reflectivity of the water surface and we are measuring the signal strength of the second echo return. I would not recommend combining hardness values from different equipment setups, but we do have some users who claim to have achieved useful results doing this, with some work on the cross-calibration.

Matt,
We use the same chartplotter/transducer across all logs. If we did manage to calibrate hardness between logs with scale and offset, would the User Map combine the Map Projects' data into one uniform scale?

Thanks,
Mike

Hi Mike,
The values in the user map/Shapefile export are unchanged from the project, so if you can calibrate them then you shouldn't have any issues using those values directly.