Georegistering Aerial Photographs: Measuring Seagrass Prop Scarring

Seagrass Submerged flowering plant found in shallow marine or estuarine waters Provides habitat for many commercially important animals Disturbed by brown tide, low water quality, dredging, and propeller scarring

Project Goals: Obtain unregistered aerial photograph Obtain DOQQ’s Use basemap with known coordinate system to register photograph Use registered photograph to digitize and measure propscarring

1. Aerial Photograph 1:9,600 format Warren Pulich Jr., Ph.D. International Institute for Sustainable Water Resources Texas State University San Marcos, Texas Coastal Bend Bays & Estuaries Program

2. Digital Orthophoto Quarter Quads Computer generated image of aerial photograph with features represented in true geographic positions Used as base map Coordinate system:NAD83 Projection: UTM http://www.tnris.state.tx.us/digital.htm

DOQQ’s: Estes Flats

Texas Coast Estes Flats, North Redfish Bay

Registering Aerial Photos Add unregistered aerial photo to basemap Use Georeferencing tools to align aerial photo with basemap Fit to display Add control points Choose transformation order

1st Order Polynomial Allows photo to be shifted or scaled, but does not deform image Requires at least 3 links

2nd Order Polynomial Allows some distortion of photo Requires at least 6 links

3rd Order Polynomial Requires at least 10 links Allows maximum distortion of photo when links are poorly aligned

Registered Map 2nd order transformation 13 links

Measuring PropScars ArcCatalog: Create New Feature Class Use editor and sketch tool to trace along each prop scar Open attribute table of New Line feature class and sum total length of new lines

Digitized Propeller Scars Total length of prop scarring = 6,456 meters Average length of each prop scar = 122 meters

Conclusions Higher resolution photo needed for measuring smaller scars Thanks. Questions?