Glossary

Term	Definition
Absolute accuracy	How close a mapped position is to its real-world coordinate. Consumer GPS typically limits this course to meter-level absolute accuracy.
Bathymetry	The measurement of underwater depth across an area. Produces a depth map analogous to topography on land.
Baseline (B)	In photogrammetry, the physical distance between successive shutter clicks required to achieve a target overlap.
Beam angle (θ)	The angular width of a sonar's acoustic beam. Narrower beams give smaller footprints and higher resolution; wider beams cover more area per ping.
Cartography	The study and practice of making maps — representing spatial information (here, the shape and features of a reef and its surroundings) in a measured, scaled, repeatable form. A Reef Cartographer is a maker of reef maps.
Change detection	Comparing maps or models of the same site made at different times to reveal what has changed — reef growth, recession, bleaching, storm damage, or recovery. The signature purpose of this specialty.
Checkpoint	A known reference point that is NOT used to align the model. It is held out to test how well the registration worked.
Cleaned data	A working copy of data after documented removal of false returns, bad points, or unusable images. Always preserved separately from the raw data.
Control point / GCP (Ground Control Point)	A known reference used to align or georeference a model. At least three non-collinear points are preferred for 3D registration.
Convergent geometry	A photogrammetric capture pattern where the cameras point inward toward a common subject from different angles. Required for robust 3D reconstruction of a discrete object.
Critical angle	The angle of incidence beyond which light traveling from a denser medium into a less dense medium undergoes total internal reflection and cannot cross the boundary. For water-to-air, approximately 48.8°.
Datum	The reference frame used for positions, depths, or elevations (e.g., WGS84, MLLW). Datum mismatches can make otherwise good data appear shifted.
DEM (Digital Elevation Model)	A digital representation of a continuous surface, used here to mean the colored 3D contour map produced from bathymetric data.
Footprint	The area of bottom illuminated by a single sonar ping. For a downward-pointing conical beam, this is a circle of diameter D = 2d tan(θ/2).
Georeferencing	Assigning real-world coordinates to data so it can be placed correctly on the Earth's surface.
GPS (Global Positioning System)	Satellite-based positioning system providing latitude and longitude. Consumer accuracy is roughly ±3–5 m horizontal.
Hydrographic survey	A professional-grade depth survey of a water body, typically conducted with multibeam sonar and RTK GPS, achieving centimeter-level accuracy. What this course is NOT.
ICP (Iterative Closest Point)	An algorithm for aligning two 3D datasets by iteratively finding closest-point correspondences and computing the rigid transformation that minimizes their separation.
Intervalometer	Camera function that automatically triggers the shutter at a fixed time interval. Used to achieve consistent photogrammetric overlap without manual shutter timing.
Nadir	The direction straight down. A nadir-pointing camera looks directly at the bottom, as in the Dive 3 orthomosaic pass.
NDL (No-Decompression Limit)	The maximum time a diver can spend at a given depth without requiring decompression stops on ascent.
No-contact rule	The absolute rule of this specialty: no living substrate, artifact, or organism may be touched, moved, or disturbed for any purpose, including improving a map or image.
Orthomosaic	A composite image built by stitching together many photos taken from above, corrected so that all pixels represent a true overhead view. A 2D product, not 3D.
Overlap (O)	The fraction of one photo that also appears in the adjacent photo. Photogrammetry typically requires 70–80% forward overlap and 60% side overlap.
Photogrammetry	The recovery of 3D geometry from a set of 2D photographs taken from different positions.
Raw data	The original, unedited output from the sonar or camera. Preserve it before cleaning or processing. A course product is not acceptable if the raw data was overwritten.
Refractive index (n)	A dimensionless number describing how light slows down in a medium relative to vacuum. Water ≈ 1.33, air ≈ 1.00.
Registration	The process of aligning two datasets (point clouds, meshes, images) into a common coordinate frame.
Relative accuracy	How consistent measurements are within a dataset, even if the whole dataset is shifted from its real-world position. Can be sub-meter even when absolute accuracy is several meters.
RTK (Real-Time Kinematic)	A satellite navigation technique using ground reference stations to achieve centimeter-level GPS accuracy. Not in scope for this course; mentioned for context.
Scale bar	A calibrated object of known length placed in the photo set to give the photogrammetric model an absolute size. Without one, the model has unknown scale.
Snell's Law	The relationship n₁ sin(θ₁) = n₂ sin(θ₂) governing how light bends at the boundary between two media of different refractive indices.
Surface Data Operator (SDO)	The team member responsible for the Fish Deeper data capture — either monitoring topside (Configuration A) or performing the seal-and-verify before deployment and the post-dive check (Configuration B). Distinct from the Surface Watch role.
Surface Watch	Topside person maintaining line-of-sight to the float and surrounding water, warning off vessel traffic, and initiating emergency plans if the float is lost or threatened.
Surveyor's Pivot	The course-specific procedure for executing a clean lane turn while towing a surface sonar float without distorting the grid geometry.
Thermocline	A sharp temperature boundary in the water column. Thermoclines can reflect or scatter sonar signals and produce false depth returns.
Tie point	A feature visible in multiple photogrammetric images that the software uses to triangulate 3D position.
Total internal reflection	The optical phenomenon where light traveling from a denser medium into a less dense medium is entirely reflected back when its angle of incidence exceeds the critical angle.

Generated from the Reef Cartographer Student Handbook, version 05.008. The course documents are the authority; if these pages and they ever disagree, the documents are right.