Marmara Island TR - Modifying SRTM v4 TIF with EVS Precision Coastal Vector File

Turkey - SRTM with EVS Vector Modifications

The map of Turkey on the left is a portion of a 600-MB World Map created by Tom Patterson. His excellent and informative website, Shaded Relief, contains a wide variety of shaded relief maps. He explains how you could create your own shaded relief maps. I have been content to download his finished maps and use them in my classroom. The students love working with them.

I have been a long time user of SRTM data. Using tools within Global Mapper, I was able to generate interesting 3D views of islands. Last week I came across SRTM v4 data at CGIAR Consortium for Spatial Information (CGIAR-CSI), which I call Cigar (See-gar) for short. I downloaded a number of SRTM TIF images covering Turkey, where I am plotting Roman era towns and cities. Using the Shaded Relief Natural Earth 1 world map I was able to plot at scales of 1:1500000 to 1:1000000. When I attempted to work closer the map became a screen full of pixels. What to do?

Marmara Island - SRTM EVS Modifications Before and After (1:25,000)

The SRTM v4 data offered me some intriguing possibilities. First, I was able to zoom into areas at scales between 1:500,000 down to 1:50,000. At 1:50,000 I had to contend with pixels. The vertical side of a pixel in the part of the world I am working in is 90 meters, the horizontal side is 70 meters. Clunky looking when working at 1:50,000. It was then that I had my "ah-ha" moment. I loaded Landsat imagery on top of the SRTM imagery. I then digitized shorelines using the Landsat imagery (section 1). I switched off the Landsat imagery and my EVS precision shoreline cut through SRTM v4 pixels (section 2) and allowed me to determine the shoreline on the SRTM image. The SRTM modified shoreline is now useful to scales as fine as 1:25,000 (section 4).

I know! A 14.5 meter pixel defining a 90-meter pixels leads to potential problems. However, for my purposes, plotting Roman era towns and cities, these SRTMs modified with EVS precision shorelines work just fine.

Marmara Island Vicinity - SRTM with EVS Modifications Before and After (1-250,000)

And at 1:250,000, they look great!

A word about my custom hypsometric tints. The colors I initially took from the Shaded Relief website. But I have since tweaked them a number of times. The current version, certainly not my last, looks great for this part of the world.

I am slowly, but surely working my way around the shoreline of the Sea of Marmara, a place I spent time as a young man. I do hope this was both informative and interesting.

Enjoy!

Coronado Islands MX - For The Birds and Fishermen

View Larger Map

Coronado Islands - Spot Image from Google Earth (1-35,000)

Coronado Islands - Other Precision Map Using Spot Image From Google Earth (1:35,000)

Located about 25 kilometers south of the entrance to San Diego bay, these Mexican islands are often mistaken as US possessions. Most San Diegans know of these islands and a few of the stories associated with them. For instance, we know that there was a gambling establishment on South Island that was a going concern well into the Depression. We know that the islands were used as a haven for Prohibition smugglers. Today there is a small Mexican military detachment stationed on South Island to keep an eye out for human smugglers and fishing poachers. The islands are visited daily by fishermen, divers and numerous sea birds, especially brown pelicans.

I first saw the islands as a young boy back in 1954. From that perspective they appeared to be two buffaloes. To others they look like coffins. To me they are always a welcome site.

The map was constructed from an image mosaic I created using the Spot image found in Google Earth. From this georeferenced mosaic I was able to work at a scale of 1:5,000. As a result, the shoreline details are excellent. I tried using SRTM 30-meter to generate contours and the results were not satisfactory. I knew of a series of geologic maps of the islands containing 20-meter contours on a San Diego Geology website. I georeferenced the contour containing images and copied the 20-meter contour intervals. When necessary, the contours were adjusted to align with the underlying Spot image.

Because my digitizing work was done at a scale of 1:5,000, the zoom factor for these islands gives one excellent details. I would suggest using Google Earth imagery to take a detailed tour of the islands. To enjoy an excellent map of the islands, this is the place.

Enjoy!

Thoughts - Generating Reliable Contours from SRTM Data

Isla la Plata - Other Precision Map (1-20,000)

Here is a comment and question concerning my use of SRTM data...

I am working on a similar project, in some aspects, about generating contours, and was wondering when the contours do not line up with your coastline (having 25 m intersect the shoreline of your polygon) do you alter the polygon or the raster data before generating the contours? Anyways, I think you work is beautiful, keep it up.

Here is my response...

Thank you for your kind words. I love making maps of islands and to be able to add meaningful contours is a great benefit. However, my experience with the SRTM 30-meter and 90-meter data is as follows. After loading both a georectified image and an SRTM file into Global Mapper, I am able to quickly generate contours at an interval I deem appropriate for the scale I am working with. In nearly every instance the lower level contours do not conform to the actual imaged shorelines. I end up manually realigning the contours to best match the imaged shoreline. As a result my lower level contours are reasonable, but suspect.

When I am dealing with a very large void that cannot be handled by combining SRTM 30-meter with SRTM 90-meter data or be filled with existing void-filling software (I use SRTMFill), I end up guesstimating alignments of contours. Consequently, all of my maps containing contours are as good as the underlying SRTM data - sometimes excellent (upper elevations of Isla la Plata map) and less reliable (10-meter to 30-meter contours and northern contours due to a void).

I am not practiced enough to correct the underlying SRTM ASCII data, so I rely upon working with software (SRTMFill), imagery to contour reconciliation, or when cloud-obscured, an educated guess using secondary imagery.

Hopefully, this answers your questions. Perhaps you have techniques or know of SRTM void-filling software that might correct many of my issues concerning misaligned or voided elevation data. I do believe that SRTM 30-meter and Landsat ETM+ at 14.5-meter resolution can work reasonably well together. However, combining SRTM 30-meter with 1-meter resolution imagery most often creates many slight, but consistent contour misalignments.

SRTM elevation data, as stated above, gives us a reasonable idea of the topography of the underlying land. I would use it to go hiking, but I sure would not bid a construction job based on it.

Enjoy!

Musandam Peninsula OM - 50-Meters Contours Using Plans One Through Four

Musandam Peninsula - EVS Precision Map (1-175,000)

I posted the above map of the Musandam Peninsula a few weeks ago. The geography is fascinating. However, I knew more work had to be done to complete my map. Specifically, I wanted to create contours to overlay onto the area.

My first attempt, Plan One, was straightforward - download the SRTM HGT file, load it into Global Mapper, generate contours and I would be finished . . . Whoops! Not so. It turns out that the SRTM HGT file for this area is full of voids, those nasty little holes of nothing. With voids my generated contours were grossly inaccurate.

I've been at this mapping business long enough to know that if one source doesn't work, there is most probably another, like Plan Two - the CGIAR-CSI SRTMs. These are void filled versions of the HGTs. I downloaded the appropriate SRTM tile, loaded it, generated contours and . . . Whoops! Not quite right.

Still not to be deterred, I went to Plan Three - using a void-filling piece of software called Void Killer 2 I tacked the contour generating problem once more. I am aware of a number of freeware SRTM void fillers, but I wanted to try the trial version of VK. I downloaded the trial copy, unzipped it and loaded the requisite SRTMs . . . Whoops! Won't let me make a copy and I didn't want to spend $43 to test it's claim as the best void filling software.

Musandam Peninsula - EVS Precision Map with 50-Meter Contour Intervals (1-175,000)

Plan Four to the rescue. In order to use Void Killer 2, it was necessary to convert the CGIAR-CIS SRTM tile from TIFF into DEM format. I loaded this DEM tile on top of the void peppered HGT file and generated contours . . . voila! It worked. Somehow the combination of the two sandwiched together allowed GM to generate contours without voids. Some of the 50-meter contours spill across water versus land, but that can be cleaned up. The HGT voids were filled with reasonable contours. The combination of SRTM HGT and SRTM DEM did the trick.

I would caution the state of Oman - Gentlemen, don't embark upon a multi-million petro-dollar project based upon these contours. However, I am confident that GM's contour generating algorithm is sound for the intrepid Omani backpacker, who could rely on these contours with a fair level of confidence.

Enjoy!

Comparison - WVS, SWBD, NGA PGS Global Shorelines and EVS Precision Shorelines

Three Vector Shoreline's Compared - WVS, SWBD and EVS Precision

One of my readers suggested I evaluate SWBD shorelines in comparison to other freely available shorelines. Previously, I compared these shorelines with my own EVS precision shoreline. I'll do it once more.

The WVS is widely available and manageable in size. There is another popular version of it called the GSHHS. It repairs varous WVS anamolies. In addition, it conveniently breaks the dataset into land, lakes, ponds and islands. At only 56MBs zipped, it is a very attractive option. As one can see in the two images, the WVS is the least precise of all large scale global shorelines. It is designed to be used at scales of 1:250,000 and above. For most of us designing projects for our classroom, employers or customers, this dataset does a fine job, it is free and the size is reasonable. It is when we need to do large scale precise shorelines that we require the next best global shorelines

Although the SWBD is a quality product and provides the end-user with precise SRTM water-defined shorelines, it tracks the water. The SWBD attempts to identify the place where the water meets the shore. That should be the shoreline, but it isn't. The top image shows the SWBD shoreline tracking the reef edges and ignoring the numerous motus within the reef. For us island mappers, the SWBD is not precise enough to identify the many shoreline features we require.

The same can be said for the NGA PGS shorelines. They are extremely precise, using Landsat ETM+ imagery as their base imagery. This is the same imagery I use to construct my EVS precision maps. I have the NGA PGS shoreline loaded into Marplot, a mapping package I use. The dataset is divided into 24 separate groups. My global coverage, using this dataset, is outstanding - except where clouds or other atmospherics obscure the shoreline or the shoreline consists of water-inundated areas (marsh, swamp, mudflats), which the NGA PGS product didn't identify. The cloud obscured areas on Landsat ETM+ imagery are validly poorly mappable to unmappable. The water-inundated areas are mappable, but the NGA contract called for the exclusion of these shoreline features, thus they were not mapped. I beg to differ. This is shoreline that must be mapped. There are massive river deltas NGA PGS mapped that depict shorelines that look nothing like the original Landsat ETM+ image, which is full of features that the NGA PGS product did not map. They just happen to be water-inundated. They are features you and I would describe as shoreline. Make no mistake, when the shoreline is cloud-free and not water-inundated, which most Landsat imaged shorelines are, this global shoreline dataset is outstanding. It's just all of those river delta islands that they didn't map that I have a problem with.

Four Vector Shoreline's Compared

However, my EVS precision shorelines are pretty outstanding as well. When one compares my hand-drawn shorelines to programmed color tracking shorelines (or many other automated processes), my hand-drawn shorelines track quite precisely. Something about using one's brain processing powers, manual dexterity and years of experience that consistently allows one to create more precise shorelines than the WVS, SWBD and NGA PGS products.

As I work with Landsat ETM+ imagery and learn more about the digitizing craft, I am certain software is available that could do what I do more accurately, faster and in much less time. But it wouldn't be as much fun! Would it?

Enjoy!

Thoughts - Landsat Island Image Mosaic

I have over 600+ Landsat ETM+ Orthorectified Mosaics that two summers ago I patiently downloaded and copied onto a series of DVDs. These mosaics cover all of the shorelines and oceanic islands throughout the world. It is an impressive collection. This collection, coupled with Global Mapper v9.01 (GM), allows me to load and view the individual mosaics as I desire. Each mosaic, even compressed in MrSID format, is typically 100 to 250MBs. Them is some big files!

Over a year ago I had a discussion with a reader who wished to work with these files to make his own maps. I described where he could download the files. But, alas, he said the files, even in MrSID format, were too large for his system. Was there a smaller file of just individual islands, he asked. Not to my knowledge, I guessed and went back to making maps of islands.

That was until this past month and my work in image mosaics. First I worked with DigitalGlobe imagery. Sadly, they have never responded to my product idea. Oh, well, so goes life. Lately, I've been working with Landsat ETM+ again. And the following is my latest idea - Landsat Island Image Mosaic or LIIM.

Greek and Turkish Islands - Landsat ETM+ Mosaics

Some of my earliest mapping efforts were of Greek and Turkish islands located in the Aegean Sea. This area is virtually cloud-free and the clarity is outstanding. However, it takes seven Landsat mosaics to cover the area. Using a raster export feature within GM, I experimented with various raster formats and came up with georeferenced JPG. Pixel size of each resulting LIIM is 14.5-meters, 300 DPIs and projection is Mercator. The resulting image is outstanding.

Nisos Limnos - Landsat ETM+ N-35-35_2000

I zoomed into my target area encompassing the primary island and created the georectified JPG. The above Nisos Limnos LIIM taken from GM gives the user the complete resolution power of Landsat ETM+ without the size problems. The above file, along with it's accompanying DEM, is only 7.2MBs.

Greek and Turkish Islands - SRTM Tiles

To really bring the Nisos Limnos LIIM to life, the SRTM data is the ticket. To cover the Aegean Sea one needs 83 individual SRTM tiles. Loads of data for most of us. Many of these tiles require some fixing, which is done using SRTM Fill (freeware) to fill holes. Once this data is loaded, GM allows for DEM creation. One is able to take the Nisos Limnos DEM and drape the Nisos Limnos LIIM on top and get a variety of 3D views of the island.

Nisos Limnos - SRTM N039E025

So I exported the underlying area into DEM format. This allows for some nifty looks at the island

Nisos Limnos - 3D View

Pretty cool! This is the Nisos Limnos LIIM draped over the Nisos Limnos DEM. All of this wrapped up in a 7.2MB package. I like it. How about you?

I should add that I have created 16 Greek and Turkish Island LIIMs. The following islands have LIIMs - Bozcaada, Gokceada, Nisida Fournoi, Nisida Gioura, Nisos Agios Efstratios, Nisos Alonnisos, Nisos Chios, Nisos Ikaria, Nisos Lesbos, Nisos Limnos, Nisos Pelagos, Nisos Samothraki, Nisos Skiathos, Nisos Skopetos, Nisos Skyros and Nisos Thasos. I intend to complete coverage of the islands of the Aegean before I head back to school on Jan 03, 2008.

Enjoy!

Isla del Coco CS - Treasure Island

Isla del Coco

1) The first image is DigitalGlobe's (DG) coverage found in Google Earth (GE). Using GEs Path Tool, I digitized over 3/4 of the islands shoreline. The digitized shoreline from a DG image is considered to be EEVS precision, meaning scales below 1:12,500 is routinely possible. Once I had completed my GE phase of this project, I saved the completed EEVS shoreline with cloud-obscured gaps identified for later work. I was able to then load this KMZ file into Global Mapper (GM)

2) The balance of the shoreline was digitized using WRS imagery, specifically those identified as path 016 row 056. The imagery is packaged in 257MB TAR files. Once downloaded and unzipped, one can select from a number of TIFF images. I used nn40 tif image.

3) With the shoreline complete, I systematically added layers of information (name labels, land polygon, streams, contours). The SRTM image, N05 W088, served a double purpose. First, I was able to use GMs "Generate Contours" routine to create contours. In addition, The SRTM image was used to create a shaded relief image of Isla del Coco.

Isla del Coco - Island Image Georectified JPG

Now for some data for all of you with mapping packages. I have zipped 9-layers of data that makes up the vector files for this project. The layers include: shoreline, streams, contours, name labels. Using GM, the zipped shapefiles load in about 2-seconds.

Isla del Coco - Vector Shapefiles

The second package contains a georectified JPG imag of the island and a DEM formatted file of the island. It should be loaded first with the JPG draped over it. You can do some neat 3D work with this combination of images.

Isla del Coco - Georectified JPG and DEM Files

Isla del Coco - 3D View SW to NE

If you load it with various vector layers, you can see them draped over the landscape of Isla del Coco.

Now for the "Treasure Island" stuff. According to Wikipedia, there are at least 3 treasures buried on this island. Pirates used the island as a hideout for years. Supposedly, Robert Louis Stevenson used Isla del Coco as inspiration for Long John Silver's Treasure Island.

Enjoy!