Comparison of Four Different World Vector Shorelines
If you look at most commercial digital atlases, their data is based on Operational Navigation Chart (ONC) data, which was originally compiled and maintained so that pilots could use these maps as reference tools when viewing features from 20,000 feet. They worked well for their intended purpose and were reasonably maintained. They were largely fashioned from pre-satellite data, much of it derived from aerial photography dating from WW-II through pre-Vietnam. Any cartographer worth their mapping "salt" will tell you that much of the data is, at best, only moderately accurate. ONC shorelines are close to actual shorelines, but not very close. The ONC shorelines were never intended to mirror the actual shoreline. I mean the scale is 16-miles to the inch!
So from this data source a product called Digital Chart of the World (DCW) was produced. Every atlas company producing small scale mapping uses some or all of the DCW data. Since my area of focus is global shorelines, I can state with certainty that if DCW shorelines are used in a map product, those shorelines are only moderately accurate. But for many map users, moderately accurate works. My Michelin maps got me from city to city and allowed me to navigate with a solid degree of certainty.
World Vector Shorelines, at a scale of 1:250,000 (4-miles to the inch), were fashioned from Tactical Pilotage Charts and other similar sources. Their shorelines are more accurate than DCW/ONC shorelines. However, they also more often miss the shoreline than track it.
Landsat ETM+ 2000 Coverage
Now we get into the realm of space-based mapping. With a virtually cloud-free image of earth, Landsat gives us a georectified imagery base from which extremely precise shorelines can be extracted. And it was done! The Prototype Global Shoreline, using a carefully researched, crafted and tested digitizing algorithm created a global shoreline that can be used at between 1:125,000 to 1:62,500 scales. If DCW/ONC shorelines are moderately accurate and World Vector Shorelines are extremely accurate, Prototype Global Shorelines track the shoreline with a high degree of reliability. That is until one zooms in and studies the PGS vectors as they relate to the Landsat image from which they were derived.
EVS and PGS Vectors
Upon closer inspection, they do not track the shoreline very often. They are typically anywhere from 15-meters to 200-meters off-center. Their vectors have a habit of "stuttering" unneccessarily as can be seen in the above image. They also do not account for large sections of shoreline that is water innundated, per NIMA's request.
EVS Would Digitize the Water Inundated Areas
EVS vs PGS Precision
Finally I get to my product, Enhanced Vector Shorelines (EVS). All of my shorelines are done by hand. The vector plotting errors inherent with this methodology are minimal compared to PGS errors. EVS provides the most accuracte shorelines extracted from Landsat ETM+ 2000 mosaics. Unfortunately, that only includes the coast of Morocco, Taiwain, Madagascar, Baja California, Caspian Sea and many islands scattered throughout the world.
Now to repeat the question I posed at the beginning of this post, "Who cares?" Should the world shorelines be mapped at EVS precision? Is the PGS, which is pretty good, sufficient for most projects? Is pretty good, good enough?
I think that a world shoreline at EVS precision would be extremely useful. I believe, that properly constructed, it would become the base mapping data for military, government and private sector projects for years to come.
I know I have a large number of GIS savy readers. What do you think? Should the world be mapped at EVS precision? And if so, why? Feedback would be appreciated.
Enjoy!
Labels: DCW, digitize, EVS, Global Shorelines, Landsat, ONC, PGS, SRTM, WVS
