How To Realign Misaligned Contours Using the "Eye-Ball" Method

Rapa Iti - NE Coast GeoTIFF Image False Color (1:10,000)

I have been working on a redo of Rapa Iti Island, using an exquisite, cloud-free GeoTIFF image, for the past month, an "on-again, off-again" project. I have completed the island's revised shorelines taking them from EVS precision to EEVS precision. That is from Enhanced Vector Shorelines using Landsat ETM+ base imagery to Extremely Enhanced Vector Shorelines using 1-meter resolution base imagery. The "zooming" capabilities of EEVS are pretty amazing. If you map it, one could zoom in to view your backyard which is in need of mowing.

Anyway, the revised Rapa Iti shoreline is finished. I still have a variety of vegetation layers to digitize, along with human ground signatures (dwellings, roads, paths, harbor features, etc). Before I tackled these challenging layers I thought I would quickly generate topographic contours. Using Global Mapper's "Generate Contours" routine, one merely needs to import the appropriate SRTM image for your area of interest, select a few options and one has contours generated in seconds. These contours, when SRTM is accurate and complete (no voids), accurately reflect the elevations of your target area. But, SRTM tiles often have little voids, which can be quickly filled using software tools (my personal favorite is SRTMFill) and in certain areas, big voids, which are difficult to reliably fill. After much thought, research and counting my money (I hate to spend money on pricey software), I decided to use a tried-and-true method "eye-ball-it". Any cartographer worth their salt cringes at this method. Too often, unscrupulous map-makers foist their finished product off as a rigorously crafted map following standard cartograph protocols, when, in fact, the map is more fiction than fact - none of which speaks kindly of the "eye-balling" method. However, since I do not have the means to obtain surveyed contour lines or modified SRTM data of my target island, I'll just "eye-ball" this portion of the project. This involves looking at the underlying land features and comparing them to accurate, reliable contours, guess-timating possible alignment of the various misaligned contour tracks (i.e., 20-meters, 40-meters, etc) and finally realigning them.

Rapa Iti - NE Coast Misaligned Contours Realigned (1:10,000)

Let's begin with Maputu Pt or the NE Coast of Rapa. Because the SRTM data was incomplete, containing a large void (no data or incomplete data), this point had contours that were spilling into the ocean.

Rapa Iti - NE Coast SRTM Base Image Misaligned Contours Realigned (1:10,000)

The above image shows the SRTM limits as a darker shade of green, when, in fact, the actual shoreline is west of the SRTM shoreline. Using the GeoTIFF image, I was able to realign the misaligned contours (see the blue contours). The method I used is "eye-balling". I looked at the shape of the easterm side of the point and beginning with the highest elevation, I realigned each contour level. The eastern side of Maputu Point appears to be quite steep, based on the GeoTIFF image and my contour realignments.

Rapa Iti - NE Coast EEVS Map with Realigned Contours Draped over SRTM Image using GM's 3D Effect (1:10,000)

Using Global Mapper's 3D Effect, I was able to see my work in a different, more revealing way. I think my "eye-balling" of the misaligned contours works quite well.

Rapa Iti - NE Coast GeoTIFF Image False Color (1:10,000)

From my GeoTIFF image to . . .

Rapa Iti - NE Coast EEVS Map Misaligned Contours Realigned (1:10,000)

Rapa Island, Maputu Point using EEVS precision (hi-res imagery). The green contours are generated from SRTM data. The blue contours are my realigned contours. They track the elevations more accurately then the SRTM based contours.

Rapa Iti - S Coast GeoTIFF Image False Color (1:10,000)

Rapa Iti - S Coast Misaligned Contours (1:10,000)

Along with the misaligned contours in the NE, another area of misaligned contours is along the southern shore. Looking at the GeoTIFF image without contours, one sees a coastline fronted by some pretty steep south facing slopes. Looking at the contours generated from the SRTM data, it is very easy to locate the problem contours. The steep slope is either poorly defined or nondefined by contours. Based on SRTM data, it appears the immediate shoreline is a very steep cliff fronting the shore. The contours do nothing toward resolving the remainder of the slope. Also the point that extends out is not defined.

Rapa Iti - S Coast EEVS Map Misaligned Contours Draped over SRTM Image with 3D Effect (1:10,000)

Looking at the contours draped over the SRTM image the reason for the misalignment is obvious. Where there should be elevation data there is none.

So I will carefully "eye-ball" the slope and assigned contours where I guess-timate they belong. You know and I know these realigned contours are nothing more than educated guesses. In the case of this map of Rapa and in consideration of the quality of the base GeoTIFF imagery, the educated guesses will ultimately lead to a good map that is useful to gather an understanding of the topography of this island. If I were to have actual contours that could be applied to this map, I could produce a high quality product useful for a wide variety of purposes. As it is, my map containing "eye-balled" contours will be a quality product useful for a variety of purposes.

Stay tuned. This project should be finished within the next few weeks. Then, you be the judge.

Enjoy!

Thoughts - Easter Island Shoreline 12,615 Vectors Later

Easter Island - EEVS Map (1:100,000)

This is a continuation of my previous post. EEVS quality mapping requires lots of vectors. As the title states, at this preliminary stage, 12,615 vectors were digitized by me to create this shoreline. I worked in Google Earth using the path tool. The first pass was relatively fast taking about 6-hours. The final pass, the fine tune, took another 3-hours. I will have to work on this shoreline in Global Mapper to finish it. Two parts of the island, the north and western edges will be digitized using Landsat as they are not included in the Google Earth map. I have loads of work to do on this map digitizing layers of information.

My point is that DG imagery is great, but to execute a quality shoreline, one must do lots of digitizing.

Enjoy!

Thoughts - Landsat = EVS / DigitalGlobe = EEVS

Groupe Acteon - Landsat Image S-08-20_2000 (1:200,000)

Groupe Acteon - EVS Map (1:200,000)

Ignorance is bliss. or so they say.

Two years ago I was thrilled to make a map using Landsat ETM+ as my base imagery. These Enhanced Vector Shoreline (EVS) maps are some of the most detailed, consistently formatted and visually appealing maps freely available to any and all. I have mapped islands, employing rigorous and proven procedures, ensuring that the stage, which consists of Landsat images and EVS maps, is clearly rendered so that their individual stories can be told in their unique geographical context, as evidenced by the Groupe Acteon Landsat image and EVS map above.

Groupe Acteon - DigitalGlobe Image in Google Earth (1:200,000)

Ignornace is bliss. until you know what you know.

What used to be, in Google Earth (GE), a smudge of green suggesting land, is now DigitalGlobe's (DG) high resolution imagery. GE and DG, working together, gives us exquisite high resolution views of islands throughout the world, including Groupe Acteon. Where Landsat ETM+ used to rule, not so anymore. The new ruler is DG using GE's mapping platform. DG's high resolution imagery costs big bucks, unless you are a mega-funded government agency. Big bucks that us little folks will never be able to afford. Working with GE, this imagery is now ours to view. Oh, yes, and ours to use as base imagery to create non-commerical maps (just make sure you credit sources).

Last night, I finally faced up to a harsh reality - EVS quality pales in comparison to Extremely Enhanced Vector Shorelines (EEVS) quality which comes from DG's high resolution imagery. The improvement in shoreline resolution is dramatic. What Landsat suggests, DG clearly shows. Striving to produce the best map, I am faced with a genuine dilemma, to redo EVS quality maps when EEVS quality is available, or not. I want to say "Not.", but I realize "Redo" is, most often, the correct response. Let me explain. The islands I map are very small chunks of real estate. Consequently, little digitizing inaccuracies become very big mapping inaccuracies. Take a look at the following.

Matureivavo Atoll - Landsat Image S-08-20_2000 (1:20,000)

Matureivavo Atoll - EVS Map (1:20,000)

Digitizing from the Landsat image, I was able to create the above EVS map of Matureivavo Atoll. Considering what Landsat was able to show, in my opinion, the resulting map was a solid effort. However, Landsat, at 14.5 meters per pixel is not DG HiRes at 1 meter per pixel.

Matureivavo Atoll - DigitalGlobe Image from Google Earth (1:20,000)

The EEVS map I will create, using the above DG imagery, will be considerably different than my Landsat effort, as it should be. I will map the majority of this complex reef as being water inundated. The small sand bars will be mapped as above water features. The sparse vegetation layer along this reef will be designated as underwater vegetation and be mapped accordingly. The resulting shoreline of this section of reef will be considerably different. Much of the original EVS quality shoreline will now be mapped as an underwater feature. I will make many different mapping decisions based on this high quality image, resulting in an EEVS quality map.

The horns of my dilemma - Is there room for both EVS and EEVS quality mapping?

Obviously, when Landsat imagery is the only available imagery, EVS rules. However, when DG imagery is available for the same area, EEVS rules. As I compare my EVS maps to DG's EEVS quality imagery and I find the EVS details are of sufficient quality to qualify as an accurate map, it will remain. If I compare an EVS map to DG's EEVS quality imagery and find it lacking in detail and accuracy, I will redo it.

Enjoy!