Abstract

Some basic information on handling external GIS data in WinCan Map

Author

Steve Peregrine BEng Hons, Senior Technical Manager

Page Contents

Introduction

WinCan Map is a fully editable GIS application and some basic understanding of GIS data is needed to handle data efficiently and effectively.

Coordinate Systems

The single most important thing to understand as Coordinate Systems, sometimes also know is Projection Systems.

It is possible to plot any point on the Earth with latitude and longitude measurements in degrees, minutes and seconds.

Longitude measurements go 180 degrees East and -180 degrees West around the planet from zero degrees at Greenwich, London (known as the Greenwich Meridian) until the two directions meet exactly on the other side of the world.

Latitude measurements are made from zero at the Equator to 90 degrees North to the North Pole and -90 degrees South at the South Pole.

Typical values in Southern Germany might be 49 degrees North from the Equator and 10 degrees East from Greenwich.

This type of plotting is used by most GPS devices and is commonly known as WGS 84 (EPSG: 4326) and works all over the world, but is not very user friendly when working with the coordinates.

Nearly all GIS data asset management systems prefer to handle their coordinates in Cartesian X and Y metric metres (or feet in the USA), but the World is too big to measure in this way, and it is also not flat which is a requirement of such a system.

So, we have coordinate systems (also known as EPSG numbers) which are relatively small areas of the Earth’s planet where the curvature of the planet can be ignored, so the surface is almost flat, and within the boundaries of the coordinate system, points can be plotted very accurately starting at (0,0) in the lower left corner of the area, to (max,max) in the upper right corner of the area.

Consider our point in Germany, plotted on the world at 49 degrees North and 10 degrees East, something like this:

But, our clients want to work with this coordinate in metres, X and Y, so we have to convert this coordinate into a projection system.

There are thousands of coordinate systems around the world. Some small countries like UK and Israel are small enough that they only need one coordinate system for the whole country, but large countries like USA can have maybe 3 coordinate system in only 1 state. There is no fixed rule on this, and coordinate systems cannot be guessed. They must be known and clearly defined in order for points to be in the correct place in any GIS application.

A common coordinate system used in Germany is EPSG 25832, but there are others.

To make things even more complicated, coordinate system can overlap each other, and the result of all this is that you can have a point on the ground which is fixed, but it will have different coordinates in different systems, and the differences can be very large in terms of numerical values.

So, let’s start with our original point in WGS 84:

In coordinate system A, maybe this same point is plotted like this:

Remember - this is still exactly the same point on the ground, but already the numbers of the coordinates are very different to how they are in WGS 84 in degrees, they are much bigger and maybe they have many decimal places. These measurements are in metres now.

But also, we can have exactly the same point in coordinate system B:

Now, you see that the same point has 3 different coordinates, and only by knowing exactly the correct coordinate system can we place the point at the correct position on the planet.

It is possible to have 20 or 30 different coordinate systems that cover the same square metre of ground.

Converting points between different coordinate systems should not be done manually and can easily be done in WinCan Map.

Worked Example

We can load some data from the Vivax API online into WinCan Map, and first we see something like this:

At first look, it looks Ok, but is it?

Let’s check that it is the correct place on the planet. To do this, add an aerial background layer from the web and then zoom to the full extents of the layer. See that the points are in the middle of the World, which is somewhere in the Atlantic ocean of the West coast of Africa:

Now, we know that there is a problem, but was in the problem?

To check it out, do 2 things:

• Check out the projection system of the layer by double-clicking on the layer in the layers panel - see that it is undefined. This is not good, so we need to know how to set it.

• Select all or one point in the layer using the select tools and inspect the X and Y coordinate values in the Info Tool - see that they are 10 X and 47 Y (not very large numbers in metres) - this is approximately what we might expect in Germany in WGS 84 degrees latitude and longitude.

So, to conclude, it is most likely that this data is WGS 84 (EPSG 4326), so we need to convert this data into the same coordinate system that we are using inside the WinCan project so that the coordinates of the object align.

To do this, we export the Vivax layer into say EPSG 25832 like this:

After we Ok the coordinate system dialogue and hit the green tick, we will be prompted to ‘Save As’ our new data, which we can give any name we wish (I will use My_Vivax_Points) and the best recommended place to save this data is in [WinCan project Folder] → Module → Map.

After export, we can remove the online Vivax layer and load the new shapefile that we just created that is in the correct coordinate system and inspect the data again:

See that the points are now in the correct place and the coordinates are completely changed, because they are now in the require projection system EPSG 5832.

Now, we can plot manholes into our WinCan project by snapping them to these points so that they take the coordinates perfectly from the shapefile.

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