Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: sextant without paper charts
    From: Frank Reed
    Date: 2008 Nov 06, 00:29 -0500

    Lu, you wrote:
    "Consider then the crudeness of our charting capabilities. Satellites can't
    spot stuff below the water like they can above the water. Side scan sonar is
    great -- if you've got the time and money to scan the ocean a few hundred
    yards at a swath.  Bottom line:  we really don't know what's out there in
    the great oceans."
    This is very true. In this age of terabytre databases and mapping systems
    like Google Earth, it's hard to remember that most of the Earth's surface is
    beneath the oceans and nearly unknown. Some satellite technologies can be
    used to create pseudo-maps of the ocean's bathymetry. For example,
    satellites can accurately map small bumps and depressions in the mean
    surface of the ocean that are caused by gravitational anomalies beneath the
    sea. A seamount has its own gravitational field and piles up water above it
    in a little hill. This type of data has been used to generate a few popular
    databases which seem to map the ocean floor in considerable detail. One
    version of this database is known as ETOPO2 (a successor to the low-accuracy
    ETOPO5). The claim is that the ocean floor is mapped with a spatial
    resolution of about 2 minutes of arc. Here's a description:
    Unfortunately, this system for generating bathymetic data is only valid on
    average. Some seamounts can be detected this way, but many cannot. The basic
    problem is that mountains can exist without the associated gravitational
    anomaly. Seamounts slowly settle into hydrostatic equilibrium with the crust
    beneath them and when that equilibrium is reached, the sea surface above no
    longer shows the gravitationally-induced hills. As a result databases like
    ETOPO2 cam reveal unknown seafloor features, but they can also miss a lot.
    They also include spurious bathymetry, essentially random noise, on scales
    of 5-100 kilometers. In addition, the satellite data is constrained by some
    trackline bathymetry which leads to strange "stripes" of pits and bumps on
    the ocean floor where the tracklines over-rule the satellite data.
    Naturally, there are bound to be classified datasets which include better
    resolution of features in some areas, but the problems above are rather
    fundamental. There's no cheap way around them. Unless someone develops a
    radically new technology for surveying the ocean floor, it will remain
    largely unknown for many decades.
    "The San Francisco's captain received a career-ending reprimand because
    another chart (not the one he was using) referred to the possibility of
    seamounts in the region in which he was operating and the court of inquiry
    found he was negligent for not using "all available navigational
    information." "
    It's also a question of prudence. The trick is recognizing that the chart
    may not be complete in an area like that. In the end, the captain took the
    fall because that's part of the captain's job. A sailor died. A nuclear
    submarine suffered a hundred million dollars in damage. Then again, he and
    his crew saved the boat from mortal injury... but the captain doesn't get
    credit for that.
    Can you imagine what that collision sounded like to the sonar operators
    aboard other vessels in the area? They must have guessed the sub was lost.
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site