# Repeat Mapping
Repeat mapping is a technique in which highly resolved surveys are collected, and then repeated on a later deployment, in order to observe measurable change to the area of interest. Many geological and biological processes of interest (e.g. fault creep, sedimentation, scour by low intensity currents, growth or movement of benthic animals) can produce seafloor changes on the order of 1 cm/year. The ability to conduct repeated mapping was a primary reason why this system was developed, and those changes have driven design requirements. At the 1-m scale, this technique has been employed using data from the Dorado class AUV in the past for mapping large scale lava flows from Axial volcano \cite{caressAxial}. One example will be presented here to illustrate the usefulness of extending this analysis into the scale collected by this low altitude survey system. 

The Monterey Canyon contained within the Monterey Bay National Marine Sanctuary is a highly dynamic system of sediment transport. The active canyon is comprised of features that can be observed though these imaging efforts. Using this payload, it is possible to determine a wide array of geomorphic change. This site is of particular interest to MBARI scientists. A multi-year  operation, known as the *Coordinated Canyon Experiment* \cite{CCEwebsite} has emplaced a network of sensors in the Monterey Canyon targeted at detecting benthic events associated with canyon dynamics studies. On January 15th, 2016, an event was detected by that network of sensors.

The imaging payload has conducted surveys on this area before and after the event. Figures \ref{fig:novtopo} and \ref{fig:maytopo} represent two 5-cm resolution topography maps collected using the 400-kHz multibeam sonar before and after the incident. The benthic event in this area involved sediment transport entering the survey area from the upper canyon wall to the east and creating clear changes in the seafloor. Most notable changes are in the southern regions where there are new depressions. Overall, more texture appears to be present in November’s survey when compared to May. 

![Multibeam sonar 5cm resolution map of the 1850m Bin Site, collected in November 2015. \label{fig:novtopo}](./img/CCEBIN_20151116_Topo5cm_ShadeB.jpg){width=45%}

![Multibeam sonar 5cm resolution map of the 1850m Bin Site, collected in May 2016. \label{fig:maytopo}](./img/CCEBIN_20160509_Topo5cm_ShadeB.jpg){width=45%}

![Observed change between data shown in Figures \ref{fig:maytopo} and \ref{fig:novtopo} \label{fig:topodiff}](./img/CCEBIN_20160509m20151116_TopoDiff5cm_NoShadeB.jpg){width=45%}

Since these measurements are all made by self-consistent and highly accurate systems, we are able to quantitatively compare the two surveys. Figure \ref{fig:topodiff} depicts the measured difference in topography. Total change measures up to nearly 1.2-m of deposition, and nearly 0.9-m of erosion in some areas. 

These types of data are vital to understand the total change over this 100-m by 100-m area. This location was chosen for detailed mapping because there was a cluster of monitoring instruments deployed nearby. Thus the maps and direct physical measurements can ultimately be compared.