|NOAA Ship Fairweather in Alaska - where Mike and I lived aboard and worked for two years.|
As many of you may know, Mike and I worked as hydrographers for the past 8 years. Nearly every time we tell someone about our (former) profession we are met with blank stares -- hydro-what?
- : of or relating to the characteristic features (as flow or depth) of bodies of water
- : relating to the charting of bodies of water
- Miriam Webster Dictionary
Hydrography is the science of measuring the depths of navigable bodies of water and searching for underwater dangers to navigation, acquiring data for nautical charts and modeling the seafloor.
Be sure to watch the short video about hydrography at the end of this post.
Today being World Hydrography Day we would like to dispel some of the myths concerning the data that is displayed on your chart plotter.
- "Spending more money on chart cards for a chart-plotter will give me better data"
- Since all chart data comes from the same "source data" this is an incorrect assumption. However, some chart cards use all the detailed data available in all areas the chart card covers, while others only show detailed information in high traffic areas.
- "The company that sells my chart cards collects and updates the data for the charts"
- In the US, and most other countries, it is a branch of the government that runs the "hydrographic offices". In the US NOAA's Office of Coast Survey produces nautical charts and related products.
- "I have a fancy 3-D chart plotter and can see the seafloor as it really is."
- The data on the 3-D display could merely be interpolated from the spot soundings on the regular chart. This all depends on the age of the source data. Be careful: If the original chart hasn't been updated in the last 20 years or so, the spot soundings do not come from a survey that scanned the entire seafloor. There could be unknown obstructions in between the soundings. NOAA charts include a source diagram that gives the user the age of the surveys which were analyzed to make the chart (Chapter 1 of the United States Coast Pilot has more information on source diagrams). Here is what can happen if you rely upon a chart which was compiled from an old survey that utilized lead line for soundings: Read the article.
- "Charts should be easy to update with the advance of satellite data"
- While satellite data can help in establishing coastlines and other above water features, satellite data is not used for depth measurements.
- "Most ships and boats have echo sounders. Why can't we use all that data to update charts?"
- "Airplanes have lasers that can pierce the surface and see the seafloor"
- Laser or LIDAR data is being used for chart updating however there are many factors such as poor water clarity that lead to inconsistent results in data quality.
- "Nothing in life is free, I have to pay for my charts"
- In the US, chart data and updates are available for free (well, actually you paid for it with your tax dollars). Chart plotter manufactures take this free data, copy it to their chart cards and sell the data back to you. There is free software for your computer that will read the free US charts downloaded from NOAA as well as ocean charts from NGA.
- "Here in Mexico, my boat shows up on land, so I need to buy a better chart card."
- As a country with far less money to spend on hydrography, Mexico's charts are often outdated and inaccurate, especially in regions less traveled by commercial shipping. If you find yourself on land a lot make sure the horizontal chart datum on the chart plotter matches that of the chart card (most likely NAD83 or WGS84).
|Small "launches" are used to survey in areas the ship is unable to maneuver. It sure was an amazing commute!|