Comparison - Haifa Harbor Using 1:2000000, 1:250000, NGA PGS and EEVS Precision Shorelines

Haifa Harbor - DigitalGlobe Image from Google Earth (1:4,600)

Let's begin this comparison of shoreline accuracy by superimposing four shorelines onto a 1-meter resolution image of Haifa Harbor. Why Haifa? Just because I happened to be making a land-sea mask in this part of the world. My initial study focused on using NGA's Prototype Global Shorelines (PGS) created from Landsat ETM+ imagery. As you will read and see this global shoreline is one of the most detailed available. It does have a few negative considerations, like "stuttering" shorelines, non-mapping of water-inundated land (swamps, marshes, etc.) and hand-tracing of shorelines is more accurate than the software generated shorelines...but, that said, NGA PGS shoreline is a great first-step toward a global shoreline for all of us.

Haifa Harbor - 1:2,000,000 (Red) and 1:250,000 (Blue) (1:4,600)

The 1:2,000,000 (Red) shoreline is, by virtue of it's name, designed to be used at 1:2,000,000 or smaller resolutions. It works well for coarse shoreline delineation to get "an idea" of where the shore ends and the sea begins. The 1:250,000 goes by the acronym W(orld) V(ector) S(horelines). It was originally extracted from Tactical Pilotage Charts. It gives one a useful shoreline for a wide variety of projects. However, when one pushes it beyond 1:250,000 it quickly proves inaccurate in tracing the actual shoreline.

Haifa Harbor - NGA PGS (Yellow) and EEVS Precision (Lite Blue) (1:4,600)

Now for the good stuff! The NGA PGS precision shorelines do a fairly good job of tracking the actual shoreline. They are deemed appropriate at scales between 1:125,000 to 1:250,000. Some areas could be pushed to 1:62,500, but be careful. I am careful and I push them below 1:62,500, for digitizing purposes, in order to confirm my Landsat derived hand-made vectors are of greater precision than the PGS. And mine are! For the purposes of this comparison the PGS shorelines would give a person an accurate shoreline impression as long as it was not superimposed over the actual shoreline. The lite blue line is what I call E(xtremely) E(nhanced) V(ector) S(horelines). They are derived from DigitalGlobe's 1-meter resolution imagery found in Google Earth. Oh, that all the world were imaged at 1-meter cloud-free resolution. I feel that is a long way coming and I don't think the global shoreline vector files derived from them will be free. I suspect they will be quite costly.

Back to the land-sea mask, or what started this current study. The most accurate vector based global land-sea mask is mine. I call it GSHHS Derived Global Land-Sea Mask and you can download a copy here. Be forewarned, the file is 83.9MBs zipped and 340MBs unzipped.

The most accurate future global shoreline map should be the NGA PGS precision product, but it needs work to finish it (correct stutters, map non-mapped areas, etc.). Once it is finished, it will be THE GLOBAL SHORELINE. Until then, WVS rules.

Enjoy!

Data - Global Land Mask using Vectors instead of 1-Kilometer Grid Cells

Global Land Mask using GSHHS Modified WVS

Regular readers know that I am a long time user of World Vector Shorelines (WVS). These vectors cover the world at a scale of 1:250,000. If you download them, you will see that they are not polygons, but line files. I have always wanted to create a land-mask of all land areas within this database, but have never had the time, software or computer calculating power. That was until yesterday. A reader asked if I knew whether their was a land-sea mask at better resolution then the 1-kilometer grid cell version currently available. To tell the truth, I didn't know about land-sea mask maps. After a brief search of some of my sources I found out that the main global land-sea mask was something build back in 1996 using WVS. Instead of using vectors as the deliverable, the builder fashioned the 1-kilometer grid map.

I let my reader know that the 1-kilometer global land-sea mask he currently was using was, probably, the best going. He knew of the Globcover LULC 300-meter grid cell version coming out in the first part of February 2008 to be created and made available by ESA.

Global Land Mask - Eastern Canada and US

Curious soul that I am and having time, software and computer capabilities, I decided I would create my own global land mask. I loaded the land files taken from GSHHS, a modified version of WVS, into Global Mapper. Using a simple routine within the digitizing tools, I created polygons from lines. Simple process, but very long processing time. Eurasia took over 4-hours to process. After working at it all day and night, I came up with my vector version of a Global Land Mask. The entire file is 340MBs in size (84MBs zipped). It takes about 3-minutes to load into GM, but wow! It does look pretty cool.

Global Land Mask - San Diego with 1-kilometer grid cell (1-125,000)

The above image is at a scale of 1:125,000 (1-inch to 2-miles). The 1-kilometer grid cell is placed on the map to give you an idea of the coarseness one gets using the existing global 1-kilometer grid-cell map. Even though it was derived from WVS vectors, not using the power of vectors (infinite zooming capabilities) one encounters an unnecessary coarseness not evident in the vector-based product.

I think my vector-based product is an example of a wonderful land-sea mask.

Enjoy!