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!
Labels: Global Mapper, GSHHS, Land-Sea Mask, WVS
I'm happy to see you've done this! Very inspirational. I had been tracking your progress on the 15m dataset for a few years -- and it's always nice to come by to see how things are going.
Good work!
Posted by
Daniel |
11/26/2007 07:09:00 PM
This is just what I was looking for, but I need a shapefile. I haven't been able to figure out what format this is in. Can you let me know so I can convert it? Thanks!
Posted by
Martin |
12/03/2007 10:05:00 AM
Martin,
Look at my later post about the Global Land Mask. I have a SHP file for download in that post. Good luck.
Posted by
Mr Minton |
12/03/2007 02:29:00 PM
Ok, thanks! I just checked it, and I am having alignment issues. I have spent several days searching for something that lines up properly. So far, the only one that does is from here:
http://www.nga.mil/portal/site/nga01/index.jsp?epi-content=GENERIC&itemID=9328fbd8dcc4a010VgnVCMServer3c02010aRCRD&beanID=1629630080&viewID=Article
and it also lines up nicely with a landsat image and with some ground co-ordinates that should fall on the coastline. Unfortunately the dataset is full of gaps, so I have been looking for somthing better.
I suspect it is because I am working in the southern tip of Chile, and there may be some distortions so far south.
Posted by
Martin |
12/03/2007 02:45:00 PM
Martin,
The GSHHS/WVS is not going to align tightly with NGA PGS vectors. NGA PGS was produced from Landsat ETM+ 2000 imagery. You could always digitize your own shoreline. Download the appropriate Landsat imagery and a good digitizing program - I use Global Mapper, which handles the Landsat imagery as well. Another nice feature of Global Mapper is its ability to project using a wide variety of built-in projections. Regardless, good luck and if I can help, drop me a line with your email address.
Posted by
Mr Minton |
12/03/2007 04:27:00 PM
Thanks for the tips. I considered digitizing, but there are so many islands down there! Anyway, I did find a complete polygon file with very good detail and does line up nicely. If you are interested you can check it out here:
http://biogeo.berkeley.edu/gadm/
Thanks again.
Posted by
Martin |
12/04/2007 01:12:00 PM