Thoughts - Rapa Iti's Hydrographic Layer

Rapa Iti Island - Ikonos Image (1-55,000)

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Rapa Iti Island - Other Precision Map (Ikonos Image) Hydrography Layer (1-55,000)

I believe Rapa Iti's hydrographic layer is almost complete. I always QC my digitizing before I determine a layer finished. I am watching the Pacific Life Holiday Bowl (score Okla St 31 and Oregon 42) and composing this post. A few moment ago my mother-in-law asked what my map showed. I explained that I had mapped all of the places where water would flow during a rain storm. She thought that was interesting, but liked the colors best.

The QC of this layer will involve a final classification of the streams. They will be classified as permanent streams identified by vegetation, stream width and visible water. Any or all of these characteristics evident will cause me to classify the stream as permanent. All other streams will be classified as intermittent streams.

This system of classification will have to do short of me visiting the island for a month during the rainy season and spending the time hiking to various points about the island to study water flow. Oh well, one can dream.

Enjoy! And Happy New Year!

Thoughts - Digitizing Rapa Island's Hydrography Layer

Rapa Island, North Shore - Ikonos Image with Digitized Hydrography Layer (1:1,250)

Rapa Island, North Shore - Other Precision (Ikonos) Map with Digitized Hydrography Layer (1:1,250)

Sure, I've been busy with family, friends and 12 dozen tamales served all day Christmas day at my home. Come Christmas evening I was still entertaining guests and wishing I could continue working on my current mapping project, Rapa Island. I have mapped this island a number of times, each time not completely satisfied with the finished project. So I've decided to work on this island one more time. First I redigitized the shoreline working at scales between 1:1,250 to 1:850 using a hi-res Ikonos image. After completing the shoreline, I decided to work on the island's hydrographic layer. Initially I've classified each valley as a potential stream course. The island has a number of permanent streams fed by frequent rain storms and springs. Most of the streams I've currently mapped will be reclassified as intermittent.

Rapa Island - Other Precision (Ikonos) Map with Hydrography Layer (1:52,500)

I've mapped most of the hydrographic layer over the last 5-days, actually over the last 5-evenings. The hydrography of the western portion of the island remains to be digitized. I hope to have the layer completed within the next few days. When it is finished, I intend to digitize the vegetation layer next. I'll keep you informed as to my progress.

Enjoy! Merry Christmas and Happy New Year!

Coronado Islands MX - For The Birds and Fishermen

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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!

Isla la Plata - Ecuador's Bird Island

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Isla la Plata - Landsat Image S-17-00_2000 (1:20,000)

Isla la Plata - DigitalGlobe Island Mosaic from Google Earth (1:20,000)

A group that removes feral animals from islands asked if I could help them with a map of Isla la Plata located off the coast of Ecuador. I said sure and began to search for a map or image of the island. The only map was something that was painted on a sign posted on this island. Not very accurate, but colorful. I tried the Landsat ETM+ image and found it to be obscured by clouds at a number of points on the island. I finally looked into Google Earth and found the DigitalGlobe image that covers the island. Except for clouds over the northern tip of the island, the image is exquisite. To create the above mosaic which is 12.4MBs and 4500 x 4120 lines, I copied 24 images (1280 x 911) and pieced them together. This image was georectified using Global Mapper, Landsat ETM+ and Google Earth coordinates.

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

This is the resulting map. The parties wanting to use the map did not require a vegetation layer so all that I had to map was the island shoreline. That took about 4-hours to map, not just the shoreline, but all of the offshore rocks as well. Finally, using SRTM 90-meter and 30-meter data, I generated 10-meter contour intervals. As so often happens, the 10-meter to 30-meter contours ended up in the ocean. I spent another 4-hours adjusting contours so that they resided on the island and not in the deep blue sea.

This map is done. I hope the party using it will find it helpful. I reminded them that they could not publish this map without permission from DigitalGlobe. It is my experience that DigitalGlobe is very open to allowing maps derived from their data to be used for educational research and perhaps for the removal of feral animals from fragile islands.

Enjoy!

Desecheo Island - Puerto Rican National Wildlife Refugee

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Desecheo Island - Google Image

Desecheo Island - EVS Precision Map (1:8,000)

Last summer I came across an aerial image of Desecheo Island on the front of a PDF document produced by a group representing Desecheo Island National Wildlife Refugee. I had been asked to make a map for a person intending to do wildlife research on the island. I called a person connected with the refugee and asked if I could use the image to create a map. He said that the image was property of the Federal Wildlife Service and since the image was federal government property, I could use the image to create my map. The map was completed and delivered to the researcher. End of story. Not quite.

I was reading from one of my favorite website, 425 DX News, a publication that gives all of the latest news about intended trips to far out places that radio amateurs will be broadcasting from, like Desecheo Island in February 2009. I decided to upgrade my Deseceo Island map. The upgrade took about 2-hours, but the image was another story.

I could not find my original image anywhere on my computer. The new Desecheo Island NWR PDF document has been reworked and the island overview image was missing. The old image is shown in a Google image search, but the link is gone. I decided to look in Google Earth and, lo and behold, there is the original Desecheo Island NWR image I had mapped from. It is now part of Google Earth. The image is not credited to any source, save Google. I downloaded a Google imaage of the island and it is posted above.

Really, not a lot of drama connected to this post, just an interesting image story, as far as images go. In fact the NWR image of Desecheo Island still exists on their reworked website (see above link). It is just interesting how it is not credited in Google. That's all.

Finally, I intend to offer my Desecheo Island map to the DX'ers that will be operating from the island in the February 2009 time frame. I wish them well and hope QSLs abound!

Enjoy!

Ahe Atoll - Crusing Tourists & Black Pearls

Ahe Atoll

Ahe Atoll - Landsat Image S-06-10_2000 (1:100,000)

Ahe Atoll - EVS Precision Map (1:100,000)

Ahe Atoll occupies a favorable position within the Tuamotus. It would be a potential first landfall for sailors departing from Hawaii heading south. The atoll has a single village with a population of approximately 100 hearty souls. The other 300 plus inhabitants live on the many small motus ringing the atoll's lagoon. Primary sources of income are black pearl farming and tourism, to a lesser degree.

If you followed the above link to Wikipedia, you will notice that the article has not been modified with my island article changes. I have been too busy. Give me a day or so and I'll make the changes to the Ahe Atoll Wikipedia article.

Ahe Atoll - Lagoon Entrance

My complements to Ekrem Inoru and his excellent set of photos detailing Ahe Atoll. I have included one photo from Ekrem's Ahe Atoll set showing the entrance to Ahe lagoon. Ekrem states that there is an 8-knot flow at lagoon entrance. This would be a challenge as tides flow in or out of the lagoon. Have a look at the entire set of Ekrem's Ahe Atoll photos found at Flickr. Just follow the link.

Enjoy!

Tematangi Atoll - Updated Map and Wikipedia Entry

Tematangi Atoll

Tematangi Atoll - Landsat Image S-07-20_2000 (1:70,000)

Tematangi Atoll - EVS Precision Map (1:70,000)

Tematangi Atoll is located at the southern end of the Tuamotus. I've posted an article dealing with this atoll previously. This post contains a redo of my original map and a locator map.

This atoll is of interest to Muslims as the antipode of the Qibla is about 50-kms NW. At this precise position one could face any direction and still be facing the Qibla.

I hope you find the reworked map useful.

Enjoy!