EVS-Islands: Comparison: Landsat-Derived Shorelines and NOAA Shorelines

Recently, I had a discussion with an individual who is working on a project that is extensively remapping 5000-kms of coastline. This effort will produce a vector map of the coastline suitable for nautical purposes. His group is using a wide variety of sources to accomplish their project and about 1 million EUs for funding.

I shared my efforts at mapping shorelines using Landsat ETM+ 2000 imagery as my base. He repeated an oft heard concern, "How accurate is this imagery?". USGS and other agencies publish papers that detail the accuracy of this generation of Landsat (Landsat-7). I like to make maps, not get too involved in the issues of geodetic precision. Plus, I am a visual person and to read the pages detailing the accuracy of these images is daunting.

He suggested I overlay my maps onto raster images of nautical charts to confirm their accuracy. Good idea, but I thought I didn't have ready access to raster image charts. However, I did! All of us do! Nautical Raster Charts are available for download at no charge. With the right viewer (also available) you can check out raster images of NOAA produced and maintained nautical charts.

To check the accuracy of my mapping compared to NOAA's mapping I prepared a shoreline map of one of the motus that make up Midway Island. I imported the raster image of NOAA nautical chart 19482 and georectified it.

Midway Island - A Comparison Between Landsat ETM-Derived Shorelines and NOAA Shorelines

The Landsat-derived shorelines match the NOAA chart with great precision. The middle of the ship channel into the lagoon, as depicted on the Landsat image, matches exactly with the corresponding chart layer. I would expect to see a slight variance because of the dynamic nature of shorelines. However, the map I produced using the Landsat image is a near mirror of the NOAA nautical chart. In other words, my maps are accurate!

I do not make nautical charts and would never suggest that my maps should be used in place of a properly constructed nautical chart. My maps are excellent at depicting shorelines and various layers of on-shore and off-shore information (vegetation, manmade features, reefs, etc.). They are not nautical charts. They can be used for a variety of commercial and government applications. Anywhere a precise vector map of one of my study areas might be used, I would recommend it for that project.

Interestingly, the Midway Island chart is drawn at a scale of 1:10,000. My map of Sand Island shows excellent detail at the same scale. I have been told numerous times that the best scale I might hope for, using Landsat images. would be 1:125,000 down to 1:62,750. Based on this comparison, I suspect the scale could be reduced even further. This exercise demonstrates the enormous scale flexiblity of maps derived from Landsat ETM 2000 images.

Never again will I question the cartographic integrity of my maps. They are as good as I can digitize them. And I do a damn fine job at digitizing. My mapping products, besides being solid cartographically, well-drawn, pleasantly and practically presented, are "GOLDEN"!

Enjoy!

view Midway Island US on EVS-Islands Google Maps