Aerial maps for a Lowrance GPS unit
November 27, 2012 at 8:57am
~ Captain Paul's response to: ~ East of East ~
I have not yet had an opportunity to personally view and test the mapping chips you speak about, so I cannot make a comment on the quality or accuracy of that specific product.
I can, however, give you some information that will let you determine if they meet the normal mapping standards and if they are acceptable for your use.
Modern GPS units when using the WAAS Differential signal can usually determine a position within 3 meters (9.8 feet). To get the most out of the unit, the internal or map chip should depict an accurate area of coverage that allows the user to get the most out of his GPS unit.
First, check out the U.S.G.S. National Geospatial Program Standards and Specifications at their web site at http://nationalmap.gov/standards/
Once on the site check out their Standards for Digital Orthophotos. These Standards are standards that define a digital image of an aerial photograph in which displacements caused by the camera and the terrain have been removed. A digital orthophoto quadrangle is often referred to as a DOQ (Digital ORTHO Quandrangle). A DOQ combines the image characteristics of a photograph with the geometric qualities of a map.
In other words it is NOT just an aerial photo of the terrain as viewed from a photo taken from a high altitude airplane. These U.S. standards correct the flat photo to match the curvature and bulge of the earth at the position of the basic photo image.
This bulge in the earth was recorded when we were able to radar map the earth from near earth satellites. In layman’s terms, it is basically similar to the difference in a NAD27 (North American Datum of 1927) and the more modern WGS 84 (World Geodetic System of 1984) datum. The NAD 27 datum was a mathematical estimate of what the surface of the earth compared to WGS 84 which is the actual satellite radar mapping of curvature at a specific point. The difference between the two different datum at this latitude can be as much as 150 feet. See a description of the WGS datum at http://en.wikipedia.org/wiki/World_Geodetic_System
To get the most out of your mapping program, check the program to see if the photos in the program are ORTHO photos as specified by the U.S.G.S. National Geospatial Program Standards, AND check the date that the images in the program were photographed. Most of the GPS manufacturer’s “Approved” mapping programs are ortho corrected for marine charts, topographical maps, and aerial~satellite photo images.
You can verify the correctness of the images in the program by going to several stationary topographical features in the area covered by the map chip and mark their locations. Such a feature can be the center of a bridge at a boat channel, a stationary U.S. Coast Guard fixed aid to Navigation, that is a fixed marker not a can, nun or buoy which can be moved by a storm, etc. Do not use a point of land or other topographical feature that could have been eroded since the aerial photo was taken. Use a fixed feature that is shown on the map chip.
When at these actual positions, look at the map chip. Are you at the same feature on the displayed map chip as the actual position? They should be within a relative short distance of each other. Note the differences if any from at least three such locations. Are they acceptable for your use?
You could also move the cursor to the selected position and zoom in tight and mark or save the coordinate positions. Then navigate to that position and note if you are actually at the location on the respective map in your computer. Compare the logged in coordinates and the actual coordinates when at the site. If they are not close, the map images in the chip are not ORTHO corrected.
This does NOT mean that they are useless. They still offer a “Birds Eye” view of the area. They just don’t have the accuracy that the GPS system is capable of producing.
It will be up to you to determine if there are any inaccuracies and if so how much of an inaccuracy is acceptable for your use.
In addition, you may have to purchase several adjoining maps as there is a chance that you could “run off the map” while navigating from the point of your launch and the actual fishing grounds. Determine how many such chips you may need for your fishing areas.
Check out one of the chips out and let me know how you make out.
~ Captain Paul ~