CH Cheat Sheet 2026

The 30 highest-yield CH facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. Thermocline stratification in a reservoir creates a water quality concern because the hypolimnion (bottom layer) may become: → Depleted in dissolved oxygen and enriched in reduced metals like iron and manganese
2. In an ADCP measurement, what does the term 'ensemble' refer to? → The average of multiple pings collected over a short interval
3. What is the significance of the Rayleigh criterion in the context of multibeam sonar resolution? → It establishes the minimum angular separation at which two targets can be distinguished
4. What is peer review in CH Environmental Impact Assessment? → Quality evaluation by qualified colleagues for improvement
5. During quality control of MBES data, the term 'wobble' refers to which artifact? → Nadir-aligned noise in the along-track direction from uncompensated heave
6. The oxygen sag curve downstream of a wastewater discharge describes the relationship between: → Dissolved oxygen recovery and organic oxygen demand over distance
7. Which federal permit is typically required before conducting hydrographic surveys that involve discharge of any pollutant into US waters? → NPDES permit
8. What is the importance of continuing education for CH professionals in Environmental Impact Assessment? → Maintaining current knowledge and adapting to industry changes
9. What does 'Special Order' survey accuracy (IHO S-44) specifically require regarding full seafloor coverage? → 100% bottom coverage is mandatory using multi-beam or side-scan sonar
10. What is the significance of the 'node factor' (f) used in harmonic tidal prediction? → It corrects for the 18.6-year lunar nodal cycle's effect on tidal amplitude
11. Nonpoint source (NPS) pollution is best controlled through which management approach? → Best Management Practices (BMPs) applied at the source
12. In harmonic tidal prediction, the 'epoch' (κ) of a tidal constituent is: → The phase lag of the constituent relative to the equilibrium tide at the prime meridian
13. A hydrographer is tasked with evaluating the impact of upstream water withdrawals on a downstream estuary. Which indicator is most diagnostic? → Change in salinity intrusion distance during low-flow periods
14. On a UTM grid, what is the false easting assigned to the central meridian of each zone to avoid negative coordinates? → 500 000 m
15. Which survey technique is most appropriate for monitoring long-term changes in a coral reef ecosystem with minimal environmental impact? → Airborne lidar bathymetry combined with hyperspectral imagery
16. In watershed-scale water quality management, a load duration curve (LDC) is used to: → Compare measured pollutant loads to allowable loads across a range of stream flows
17. When overlaying two polygon layers in GIS, which operation retains only the area that exists in BOTH input layers? → Intersect
18. The speed of sound used by an ADCP to convert travel time to range is primarily a function of: → Water temperature, salinity, and pressure (depth)
19. Which documentation is required in a NOAA Hydrographic Survey Project Instructions (HSPI) to define the geographic scope and priority areas of a survey project? → The survey limits diagram and priority area polygons with associated order requirements
20. Which ADCP blanking distance parameter defines the zone nearest the transducer where velocity data cannot be reliably collected? → Blank before transmit
21. Converting raw satellite digital number values to surface reflectance is accomplished through: → Radiometric calibration followed by atmospheric correction
22. The National Tidal Datum Epoch (NTDE) currently used by NOAA spans which 19-year period? → 1983–2001
23. Subsidence of a tide gauge benchmark would cause the recorded sea level trend at that station to appear: → Higher than the true sea level rise rate (apparent faster rise)
24. A subordinate tide station's relationship to a control tide station is described by which pair of parameters? → Time difference and height ratio (or difference)
25. Which baseline data type is most relevant for assessing hydrographic survey impacts on submerged aquatic vegetation (SAV)? → Light attenuation and water clarity measurements
26. A hydrographer converts UTM coordinates to geographic coordinates. Which projection property is preserved in UTM? → Conformal (angle-preserving)
27. A 'finding of no significant impact' (FONSI) can be issued after which EIA document? → Environmental Assessment
28. The International Hydrographic Organization (IHO) publication 'S-44' primarily establishes: → Minimum standards for hydrographic survey accuracy by order classification
29. When reducing soundings to a tidal datum, a hydrographer must apply a 'tide correction.' What does this correction represent? → The difference between observed water level and the chart datum at time of sounding
30. What is the primary horizontal datum used for nautical charts produced by NOAA in the United States? → NAD 83 / WGS 84