Dateline Problem

Liz Sanderson
Liz Sanderson
  • Updated

FME Version

  • FME 2016.x

Introduction

When you reproject a line or polygon that crosses the Date Line to lat/long, many applications will display it as spread across the entire world. This is because the standard lat/long coordinate system ranges from -180 degrees to 180 degrees. Coordinates to the east of the date line will have positive coordinates near 180, while those to the west will have negative coordinates near -180.

FME has a coordinate system, LL-360, that is the equivalent of the standard lat/long LL84, but whose range is 0 degrees to 360 degrees. This coordinate system is presently deprecated, and so cannot be seen in the coordinate system gallery, but is still available by typing LL-360 directly into the coordinate system text box. Due to its range, features near the date line will all have coordinates near 180, while those crossing the prime meridian will be spread across the world.

 

Step-by-step Instructions

If your data is localized to a small section of the world, you can choose which lat/long coordinate system to use to prevent the globe spanning. However, for world wide data, you will need to reproject each feature to the most appropriate coordinate system:

  • Send the features through a Reprojector to reproject to LL84
  • Use the BoundsExtractor to find the new bounds of the feature
  • Use a Tester to find features where the western bound is less than -150
  • Send these features to a second Reprojector to reproject to LL-360
  • Use a CoordinateSystemSetter transformer to change the LL-360 features to LL84 without reprojecting

Because the two coordinate systems are essentially the same, only with different bounds, the data from both will overlay properly.

The attached workspace NoSpan.fmw illustrates the process.

 

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