EVS-Islands

How I Make a Map: Image to Map

The WVS Map of Canton Island @ 1:250,000

One of the most available and detailed world-wide coverage that is freely available is called World Vector Shorelines (WVS). At a scale of 1:250,000 the shorelines are crisp, clean representations or land features. However, when you work at larger scales, the accuracy begins to degrade significantly.

My Finished Enhanced Vector Shoreline (EVS) Map of Canton Island @ 1:75,000

My quest is to enhance WVS shorelines or any of the other global shorelines, into vector files that more nearly represent actual coastal features. I call my final product Enhanced Vector Shorelines (EVS). Using Landsat ETM+ 2000 imagery, I find my shorelines rival the best global shorelines freely available to the public.

Select the Island to Map

This is Landsat ETM+ 2000 imagery of my target island. The image is cloud-free, shorelines are crisply delineated and the colors are sharp. This image is a digitizer's dream. Feature determination will be relatively easy.

Work Begins @ 250 Meters per Centimeter (1:12,500)

The first decision, after selecting the target shoreline, is the scale to digitize. My preferred scale is 1:12,500. This scale allows for detailed tracing of features. I am able to determine shorelines, beaches and in-land areas and follow their boundaries with practiced precision.

All digitizing and preliminary map work is done using Global Mapper (GM). I find it perfect for the type of work I want to do - digitizing shorelines and other features. The price is right at about $200. The ease of use is excellent. And finally, Mike, the individual that provides customer support, is unparalleled in the industry. He is continually attending to the needs of his product users at no additional cost. He works to improve his product so that it might meet the needs of his product users.

Digitizing an EVS Sparse Vegetation Layer

I begin this project's digitization by determing the limits of a layer, in this instince, EVS Sparse Vegetation. This layer consists of vegetation that is low to the ground. There might be a few trees but the layer is mostly grass or scrub brush. The actual process of digitizing a small feature is fast - maybe 1-min is spent to outline and classifying the feature. Upon completion of the outlining, I select the appropriate layer and move on to my next target area. I typically digitize a single type of layer for an entire atoll, moving from motu to motu until the layer is complete. In this way the original layer determination based on color, location within the context of the island and appearance will typically be consistent throughout the project.

Creating An EVS Sparse Vegetation Layer

This is the GM Layer Selection screen used to designating layers. All of my layers, nearly 100, are defined by name, attributes (color, line size, symbol) and range (when they will appear or disappear on the map). As my feature classification skills increase, I find that my layers attributes seldom change. I do work on color selections. Not big changes, but slight adjustments to render a finished map more tasteful.

Give Yourself Plenty of Room to Work

As you are digitizing, give yourself plenty of room on screen. My screen is 17". It gives me loads of work area before I reach the screen's edges. With GM, once you reach a screen edge the image adjusts giving you room to complete your digitizing. You can also move the image on-screen using keyboard arrows (up/down/left/right) during the digitizing phase without stopping the operation. When you create a polygon, you must finish it before it can be saved. With a line, it can be saved as often as you desire and then combine with all lines that delineate a specific feature.

What Should I Map? How Many Layers Do You See? I Mapped Ten.

The decision on what to include or exclude during the digitizing process either creates a complete map or an incomplete map. I see ten distinct layers of information in this image. They are listed along side the above image. At the beginning of your project you should decide what features to include or exclude. That is, how to identify from your base imagery certain features in a consistent manner and how these features will be classified, then begin work. Don't begin first and classify later. It makes for lots of extra work as you reclassify.

Creating a Large Area Feature of Sparse Vegetation

Large area polygons are necessary at times. When I work large areas now, I prefer working with lines as opposed to polygons. Lines can be saved before the feature is completely digitized. My software allows me to combine lines to create a single, connected line. I can then convert the enclosed lines into a polygon (area feature). Pick your own method (polygon or line).

Time to Begin Using Lines to Delineate Features

I use lines extensively during digitizing. I feel they offer the greatest control and flexibility. I can draw them, split them and move single or multiple vectors with ease. More importantly, I can save them as often as needed and then combine lines to create my final line or polygon.

Delineating the Beach from The Raised Island

When mapping islands, Landsat ETM+ renders beaches as white strips of land along a coast. Makes sense. But islands often have lots of sand at places other than the shore. Should I classify beaches or should I classify sand? I classify sand. My layer is called EVS Land Sand. Most often this feature is where we would expect it to be, along the shore. But it can be found at many other places within the atoll. I look for a color range, pale white to white, to classify as EVS Land Sand.

Digitized Shoreline, Reef, Beach, Vegetation and Piers

Layer by layer (again, determined prior to start of digitizing) is created. Some layers are fast and uncomplicated (shoreline, vegetation and beach). Other layers are not as readily rendered (reefs, certain manmade features). Digitizing is a systematic, methodical process. Tedious and at the same time satisfying

Digitizing Reef Awash - An Area of Surf and Reef

Early on, I recognized a feature unique to atoll shorelines. It looks like reefs, but it acts like waves. It turns out that, most often, it is waves. I call the layer EVS Reef Awash. I figure it is the part an atoll's reef that is marked by breaking waves. With oceanic islands this feature is typically found only on one side of the island, that being the side with the outer reef facing towards the oceanic currents. The other side of the atoll has the same submerged reef, but not the breaking waves. I classify this part of the island's reef system as EVS Reef Submerged.

Greatest Reef Turbulence is on the Eastern Shore

For our target island, the greatest wave activity is on the atoll's eastern shore. The flow of the ocean is west to east and the island interrupts that flow. Consequently, the waves break on the eastern shore.

Combining Two Lines into a Single Line

This screen shows how lines are combined. Again, this is my favorite technique - to digiitize using lines, save often and then combine like lines into a single continuous line that encircles the limits of the feature.

Digitizing a Complex Reef Within the Lagoon Area

This island has a complex reef structure in it's lagoon. In my early efforts, I would have digitized large areas of the lagoon as areas with reefs. I now work with 12 distinct layers to define island reefs. The single most important reef layer determinant is color. Next is the reef features location in relation to the atoll's motus and lagoon. Different shades of blue represent different depths. I do not use an automated process that selects depths based on color range, I use my vision and extensive experience digitizing. Reefs are tough, but when rendered consistently they are strong enhancements to your final map.

Exporting the Completed Vector Files into Marplot

This is the step that takes your digitized data (vector files) and packages them into your mapping software format. GM gives me loads of formats to select from. I work most often using ARC/Info Ungenerate, a very simple, highly portable format.

Selecting EVS and WVS Layers to Construct EVS Map

Prior to importing the completed vector files into Marplot, my mapping software, I open the various EVS and WVS layers that I will work with. By selecting them at this point, they are open and ready for action. By opening a layer, I can modify, finalize, move, clear or delete vectors to ultimately create my finished map.

Importing the Island.dat File into Marplot

My imported project is saved as a .dat file. It is a simple ascii file consisting of lat/lon's with a few other identifiers for polygons or lines. I do not import any attributes (colors, line size, fill, etc.). These are already determined within Marplot.

Temporary Layer Awaiting EVS Layer Allocation<.p>

The vector file is placed into a layer called Temporary. I typically import projects in phases. This phase will allow me to complete the island shoreline, sand and vegetation layers. I transfer certain lines of data into various EVS layers to create my final map.

First Select a Larger Portion of the Map to Work With

I zoom into an area that I want to work. The area should contain the complete limits of specific features.

Save the WVS Shoreline to a Storage Area

Remove any unnecessary features from the screen. I move the WVS Shorelines and store them in a layer called WVS Shoreline Storage. I hate throwing something away that I might need later.

Select Area to Construct EVS Raised Reef Layer

Here, I have selected an area that will be classified as EVS Raised Reef. It is an area that would be typically uncovered at low tide and is located adjacent to the shore.

Move the Highlighted Layer into EVS Reef Raised Layer

After selecting all of the lines that make up the feature, I move the Temporary lines into their appropriate layer. I should add that, most often, I work with both Global Mapper and Marplot active at the same time. I can then switch back and forth between my image and my map to ensure I am classifying properly and including all lines that define a certain feature.

EVS Reef Raised Layer with Temporary Lines Around It

The selected Temporary lines become the EVS Reef Raised layer. The Temporary lines remain. Upon completion of the feature, the Temporary lines can be deleted.

Convert Polylines to Polygons (EVS Sparse Vegetation)

Quite often, I convert multiple lines that define a feature into a polygon. A middle step, after I select the lines, is to convert all of the lines into a polygon by converting all of the selected lines into a single polyline. This polyline can then be converted into a polygon.

Creating a New Layer (EVS Reef Middle)

This particular project required the creation of a new layer called EVS Reef Middle. The process to create a new layer is fast. Name the layer (EVS Reef Middle), move it into an appropriate group (EVS), determine it's color, fill, line, symbol and range it will be visible. And that's it.

EVS Reef Middle Will be Displayed at 40.27 KM or Less

My EVS layers show up at 40.27 km. It is my preference for the uses of my maps.

Island Land Areas and Some Reef Layers Completed

The map is nearly complete. All of the above water features are finished and most of the reef features are done. Just a few more reef features to do and I will have a completed map.

Canton Island - The Finished EVS Map

And there it is. The remainder of the reefs are in and classified.

This map is one of my favorites. The above water features are well-defined. The reefs are stunning. Like I say, map making is part systematic processes and part art.

Enjoy!

Labels: atoll, EVS, island, Landsat, map, Marplot, Pacific

Posted by Mr Minton on Thursday, November 09, 2006 at 11/09/2006 06:00:00 AM | Permalink