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Monday, 7 July 2014

Coordinate Story

Coordinates are The things that distinguish a map from a normal painting or standard graphic design. For map professionals or users, there are two coordinates we need to be aware: Geographic and Projected coordinates. Geographic refers to the coordinate system on 3D Earth (i.e. Ellipsoidal and Sphere based Earth) and projected coordinates are the coordinate system for 2D flat maps.

Hence, every map-maker needs to be careful which coordinate system to be used. Sometimes, we can get away with it because the difference is obscure in the maps. However, when it comes to accuracy, getting the coordinate system right is an utmost importance.

Section 1


After long day at work and preparing for a holiday, I went back home to relax and ready up for the trip. Then, I received a telephone call, all the way from Perth. It was professional question on coordinates and it was dealing with client data. This was the real deal. (Refer to Story of Mauritius for another example of freelance work)
Source: http://www.bothsidesofthetable.com/

Problem

The company has identified roughly 200 points on a linear road stretch. It just happened that the road has 6 lanes at maximum (3 lanes heading north, 3 lanes heading south). The 200 points were captured at particular intervals where each lane has 100 points. The company want to plot 200 points to indicate the approximate location of the 3rd lane (refer to map below). Hence, the problem is identifying the coordinates of 100 points representing the 3rd lane (Left and right). I was supplied with an Excel spreadsheet containing these 200 points.

Due to nature of this project, specific details of the location of work site will not be released. 


For Illustration purposes only. The client did not request for a map
As you can see, 4 lanes of the road has been surveyed (in darker colours). 200 points to be plotted are the lighter green and red circles.  The question is how can I find the location of 200 other points (referring to L3 and R3) ?

Solution

Anything revolving on coordinates I take it with care. I spend couple of hours finding and testing methods to find the coordinates. The rule is the 3rd point is 5 metres away perpendicular (left)  to L2 and 5 metres perpendicular (right) away from R2. In the end, I found a very simple solution.

Procedure:

1) It stems from the understanding of the width of each longitude. Assuming the surveyed locations were based on purely WGS 84 and accuracy is not of utmost importance, I found a website (Length Of A Degree Of Latitude And Longitude Calculator to calculate the length of longitude at a particular latitude. For your information, there is variation of width of longitude at different latitudes. From the coordinates that I was supplied of, I keyed in the degree in this website calculator to get my answer.

2) With this understanding above, I used Excel spreadsheet to calculate the approximate locations of L3 and R3 (refer to map above). It involved simple division and subtraction/addition

3) Now, plotting in ArcMap involved a bit of research. I embedded the Excel/ CSV spreadsheet into ArcMap. Then, I right clicked on the spreadsheet, and chose 'Display XY Data'. I mapped the coordinates in spreadsheet with right equivalent of X- and Y-axes.  Refer to Importing Excel data into ArcMap for further details. The work is done.

4) For final testing, I placed Open Street Map basemap to make sure the data is plotted correctly. The ultimate verification was done through Google Earth.

5) I converted the shapefiles of these coordinates into kml (Google Earth file) and uploaded the kml file to see the alignment. Google Earth verified the 200 additional points representing the third lane (both left and right) and they were correct in their positioning. The coordinate system I used was MGA94 (UTM system for the area).

The full name of coordinate system I used would not be released due to confidentiality of the data.

The end products were the Excel spreadsheet and .kml file as a proof.

Work Time: 4 hours (from receiving the data to submission of end products)

Contact me via the blog for further specifics and assistance/consultancy works related to mapping.

Section 2

As some of you would know, I am currently involved in massive GIS collaborative project on elections (Refer to Hiatus in Blog for details). I was in the process of creating 3 shapefiles related to election boundaries: Election precincts, State Constituencies and Federal Constituencies. For the moment, I used geographic coordinate system (WGS84) to draw the boundaries of electoral precincts and federal constituencies . 

When I came to State election borders, I chose Web Mercator as the coordinate system. Unknowingly, I spent 14 hours drawing state election borders using Web Mercator while drawing them on electoral precincts (as the base map). I did a check on QGIS which coordinate system I used for State boundaries. It turned out to be severe mismatch of coordinates between State and electoral precincts borders. I attempted to reproject State borders onto WGS84 and it disappeared elsewhere in the workspace. Hence, 14 hours of my work was wasted.

Coming to this realization, I am concerned with the coordinate systems I employed for Venezuelan Map Project. It involved multiple spatial datasets from different coordinate systems. The base map for reference uses Web Mercator. Have I aligned my lakes and river unknowingly onto Web Mercator?  Since the project is suspended at the moment, I need to spend time checking on this and work out a solution for this.

In short, as a map professional, I need to do careful planning in terms of coordinate system. Hours of work could go to waste if this planning is not done well.

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