# Fine Scale Surveys
This section will describe the manner in which surveys have been conducted. Planning and executing these surveys in a controlled manner ensures 100% coverage of the area. The 12-hr MBARI operations limit for ROV dives imposes a constraint on the survey coverage.

## Survey Design
All surveys to date have been planned based on data collected previously by MBARI’s Dorado class mapping vehicles. These data, collected typically by a 200-kHz multibeam sonar, yield maps with 1-m horizontal resolution and 0.1-m vertical precision. From these maps, scientists at MBARI identify areas of interest. In many instances, ROVs are used to conduct video reconnaissance mission to validate these areas as targets for the fine scale surveys. A site is primarily considered for these specialized surveys because the area is either unique or likely to experience a change. Following sections will discuss specific cases of these targets. 

Surveys are tailored to the site of interest, but generally follow a ``lawnmower’’ style of motion, with the platform always moving forward, turning before each line. Since the lighting around the cameras is symmetrically spread, this has little impact on the stereo imaging data when processed post-dive. In sites that are visited in repeated dives, a simple marker may be dropped central to the survey area so that repeated surveys can be more readily co-located. More optimally, a bottom feature that is unlikely to move is identified near enough to serve the same purpose as the marker. 

Cross-track coverage is limited primarily by the field of view of the lidar system, and as such lines are tightly spaced for only a small amount of lidar overlap. The limiting factor to along-track speed is the acquisition rates of the stereo cameras due to the recharge cycle of the xenon strobe system. 

![Three maps, collected simultaneously by multibeam sonar (left), lidar (center) and stereo cameras (right). \label{fig:clams}](./img/clams.png){width=100%} 

## Station-Keeping Software

MBARI software engineers have collaborated with Stanford scientists to aid in development of a station-keeping that we have employed for use in these surveys. This software acts as a ``backseat driver’’ to the ROV control set, effectively replacing the joystick control with computerized thruster control. What separates this software from similar efforts \cite{Whitcomb98towardsprecision} is an interface that allows an entire survey to be scripted by a set of relative movement instructions. An origin within the survey is identified, and once the scripted survey begins, all movements reference this point. Horizontal positions are defined as waypoints. A constant altitude is  specified, along with a desired heading. Finally horizontal speed is defined so that along-track resolution can be maintained. 

These control features encompass a basic required of information to conduct a rigorous survey at the desired scales of operation. Typically, surveys are flown at 3-m altitude. Survey speeds are 0.2-m/s. Under these parameters, a 100-m by 100-m box can be covered using 1.5-m line spacing, and finished in approximately 10 hours of operation. 

## Example Survey - Clam Community

Once surveys are collected, the data are processed using MB System \cite{mbsystem}. Within the Monterey Bay National Marine Sanctuary, there is a site at 2850-m depth within the Monterey Canyon which houses a community of chemosynthetic clams. Figure \ref{fig:clams} depicts a survey conducted at this site. The three panes provide a comparison of the capabilities of each system. On the left is the multibeam 5-cm resolution data, and bathymetry is the main feature, with the clam communities resolved as a texture. 

The middle pane represents 1-cm resolution bathymetry collected using the lidar. The clam community is better defined here, and individual clams can be observed. The third pane is the photo-mosaic of the same region. Clams are clearly visible, and provides ground-truthing to targets we can identify and measure in the lidar. The map generated from lidar data exhibits features that the multibeam sonar or the photography have failed to capture, most notably the individual tracks left by the clams as they move across the seafloor.