C. Generation of Distance Lines

Owned by Harald Sund (Unlicensed)
Last updated: Oct 16, 2015 by Tore Sannes (Unlicensed)
4 min read

Introduction

Calculating distances can be done in many different ways. Geocap uses algorithms that take into account the curvature of the Earth which provides for better accuracy and is in accordance with the scientific and technical guidelines established by the Commission on the Limits of the Continental Shelf (CLCS) for UNCLOS Article 76. Any distance can be calculated in Geocap but the main ones are:

  • 12M Territorial Seas.
  • 24M Contiguous Zones.
  • 200M Exclusive Economic Zones
  • 60M Formula Lines.
  • 100M Constraint Lines.

  • 350M Constraint Lines.

 

Note that our Maritime Zones plugin extends the Shelf package with additional capabilities for calculating maritime zones. It works directly with ArcGIS feature classes and can calculate any maritime zone from both normal and straight baselines. The calculation also extracts critical points for each zone and store them in a feature class. Read more about the: Maritime Zones plugin

Exercises

200M Line Generation

Exercise

Generate the 200M line

  1. In the folder 1. Maritime lines / Base lines right-click Baseline_Atlantis and select the command Generate 200M line...
  2. Make sure that Generate base point connection polygon is checked. The menu should look exactly as the screenshot below.
  3. Click Execute.
  4. The new line should now be available under 1. Maritime Lines/200M Lines. Click OK.
  5. Click OK to close the menu and keep the settings.
  6. In the folder 1. Maritime lines / 200M lines tick the box next to the Baseline_Atlantis + 200M dataset to display it.


Generate 200M line menu


Information on point spacing

The arc on an "envelope of arcs" in reality consist of a series of short chords. The point spacing represents the length of these chords.
In the image below you will see that a larger area will be lost (outside blue chords) if we double the point spacing on the arc.

It is recommended that the chord lengths (point spacing) are determined such that the distance between the arc and the chord never exceeds 1 meter. Below you will find a table of distance lines and the corresponding point spacing which fulfills this recommendation.

Distance line (M)Point spacing (M)
3 Nautical Miles0.1 Nautical Miles
12 Nautical Miles0.2 Nautical Miles
60 Nautical Miles0.5 Nautical Miles
200 Nautical Miles1.0 Nautical Miles
350 Nautical Miles

1.0 Nautical Miles

 

Note that not all points on the baseline will contribute to the generation of the 200M line. To visualize which points on the baseline that contributes to the 200M we can display the base point connection polygon.

Exercise

Display base point connection polygon

  1. In the folder 1. Maritime lines / Base lines right-click Baseline_Atlantis and click Display Points
  2. In the folder 1. Maritime lines / 200M lines tick the box next to the Baseline_Atlantis + 200M Connection Polygon dataset to display it.
  3. In the folder 1. Maritime lines / 200M lines right click Baseline_Atlantis + 200M Connection Polygon and click Display Points.
  4. Notice how the polygons points back to the critical points on your baseline (see image below).
  5. Toggle the display between polygon mode and wire mode with the W and S keys.

200M line with critical points on baseline in blue

It is possible to store the points which contributes to the 200M line as a separate dataset in your project. This can be done by checking the Extract critical points on the Generate 200M line... menu

Exercise

Check Table View for the 200M line

  1. In the folder 1. Maritime lines / 200M lines right click Baseline_Atlantis + 200M and click Table View.
  2. Observe the columns, Base Line, Base Line Index and Second Base Line Index. This tells you which baseline and which point on the baseline that contributed to the point on the 200M line.
  3. Scroll down and select a point that has values in both Base Line Index and Second Base Line Index. If Display marked points is checked the point should be highlighted in the graphics window.
  4. Observe that this is the crossing points between two arcs in an envelope of arcs calculation (equidistance point). That is why it is referring to two base points.
  5. Click Close to close Table View

350M Line Generation

Exercise

Generate 350M line

  1. In the folder 1. Maritime lines / Base lines right-click Baseline_Atlantis and select the command Generate 350M line...
  2. Keep the default settings and click Execute.
  3. The new line should now be available under 1. Maritime Lines / 350M Lines. Click OK.
  4. Click OK to close the menu and keep the settings.
  5. In the folder 1. Maritime lines / 350M lines tick the box next to the Baseline_Atlantis + 350M dataset to display it.

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To verify that the distances are correct we can use the Distance Calculations tool.

Exercise

Use the Distance Calculation tool

  1. Open the tool from Tools > Distance Calculations.
  2. Click YES to put Geocap in 2D
  3. Select coordinate system Mercator (all values 0)
  4. For distance calculation method, select Vincenty calculation. (Flat earth calculates in the metric values of the projection)
  5. Select Distance unit: nautical mile
  6. To measure the distance between two points, press j on the keyboard to snap to them.
  7. The distance is displayed in the lower part of the panel. Notice that vertical, horizontal and total distance is measured and that lat/long of the select point is listed.
  8. Click the X to close the menu
  9. Click 3D to get back into 3D view.