I am looking for a GPS/GLONASS fishfinder and a transducer. My purpose is a bit on the unorthodox side. I work for a small civil engineering firm that assesses and and designs countermeasures for bridges. Many of our sites are over substantial rivers such as the Rio Grande and Colorado River.

The overall goal is to be able to get accurate depth measurements from the transducer that will be logged in a gps unit. The setup will be on a small fishing boat and we will run cross sections upstream and downstream of the structure to get a representation of the channel. 

We are looking for a transducer with a narrow cone angle in order to get an accurate depth measurement. We also need a gps fishfinder that will log our location and depth as we move through the channel.

Any help in selecting the proper equipment would be highly appreciated. 

 

The smallest 2D sonar coverage I can think of offhand in an "affordable" unit is the 455 kHz of a Humminbird DI unit which is 16 degrees.

You really do not need a narrow cone transducer to get accurate depth since the center of the sonar pulse (strongest part) is where the depth readout is created.

Setting a unit up properly to give accurate depth is the most important. You want the true water depth and not the distance between the transducer and bottom. That takes offsetting the depth readout for the distance the transducer is submerged. Additionally the bottom of the transducer has to be parallel with the bottom so the center of the sonar pulse is vertical (shortest distance from transducer to bottom).

GPS accuracy would be more important that sonar so contour placement is where it should be. That takes a unit with an external GPS antenna that is located above the sonar transducer so the location and depth is at the same place.

For recreational equipment, Humminbird, Garmin, Lowrance, and Raymarine have Sonar/GPS combo units that will record sonar logs for contour map generation. You can spend tens of thousands if you want for a commercial unit that is more accurate.

I have several hundred sonar logs that I have used for contour map generation using Humminbird equipment.