## Monday, 25 July 2016

### ...or for any other reason you may wish to share your current location with your Twitter followers...

You can use this to broadcast your position for any reason whatsoever. For example, with an accompanying note such as "loads of Pokémon critters here five minutes ago!" etc...

(See my previous post for other recent additions to #ReallySimpleMovingMap)

## Sunday, 17 July 2016

### Drawing Shapes on Maps

A while ago an image was doing the rounds on social media which showed a circle with radius of 500 miles centred on Leith, thereby depicting the possible search area covered by the Proclaimers in their (rather excellent ) "500 miles" .

As is often the case one sees with shapes drawn on maps, the circle was drawn as a pure geometric circle superimposed on a Mercator map projection. This is of course easy to do, but is misleading: strictly speaking, that pure shape suggests that all the radial lines would be Rhumb-lines (i.e., straight lines drawn on a Mercator projection). Although it looks like a circle, it really isn't: when drawn on the surface of a sphere (or more accurately, an ellipsoid) representing the Earth's 3D geometry, that simple Rhumb-line circle would be distorted. Instead, a circle drawn on the surface of the sphere (or ellipsoid) should be constructed from radial lines which are arcs of Great Circles rather than Rhumb-lines (note: such a circle, constructed from Great Circle arc radii, is actually called a Small Circle). The aforementioned distortion is greater for larger circles: but even at 500 miles radius, it is evident.

The image below illustrates the point. The two "500 mile circles" are depicted: the one constructed from Rhumb-lines (which actually looks like a circle on this Mercator projection map), the other from Great Circles (which is actually distorted on the map, but would look like a circle when viewed on the surface of an ellipsoid). Both are centred on Leith (near Edinburgh, Scotland). Since distances aren't preserved under projection, I have (arbitrarily) set the northern-most point of each circle to be in the same location (at 500 miles along a Great Circle from Leith). All other points on the two shapes are different. In terms of perimeters and areas, the Rhumb-line derived circular shape is larger: perimeter 5,639 km; area 254,3000 sq-km, versus perimeter 5,038 km; area 202,8000 sq-km for the Great Circle derived shape. So, assuming The Proclaimers walk along Great Circles in their quest, they will cover a smaller area of land than if they simply drew a circle on a map and used that to navigate.

### #RSMM

I've used this "500 miles" example to demonstrate the (new) Shapes functionality I've just released within my #RSMM (#ReallySimpleMovingMap) web-app.

Here's the #RSMM link containing the "500 miles"  example. Click on it to go the #RSMM web-app. From there, you can experiment with lots of different shapes (not just circles!)

Here is a summary of the Shapes functionality within #RSMM:
• Create shapes and display them on the map. Available shapes include:
• point
• line (arbitrary number of vertices)
• circle and circular arc (portion of circle)
• square
• rectangle
• triangle
• polygon (arbitrary number of vertices)
• The shapes can be specified as being constructed from either Great-Circle segments (on a reference Ellipsoid), or from Rhumb-line segments (i.e., straight lines on Mercator projections)
• For convenience, the shape coordinates are initialised via Marker locations, and then can be fine-tuned via edit-boxes
• All visual attributes of the shapes can be customised e.g., line color & opacity, line thickness, fill colour & opacity, etc.
• Save the shapes (individually or in groups) to the FlyLogical Cloud (database) for convenient re-use later (and for sharing with the public if desired)
• Export the shapes (individually or in groups)  to KML file format (sent by email) for ease-of-use with KML-supporting apps such as GoogleEarth.
Enjoy!