Projecting your river network
Many of the tools in RivEX require your river network to be in a coordinate system that is in linear units, typically metres, but can be feet. This requirement enables RivEX to create meaningful attribution and analysis. We might talk of a site being "1km" or "300 feet" away, but never in fractions of decimal degrees! The issue with data in WGS84 is that a line will change length with increasing latitude, as you move north/south away from the equator. A line that represented 1km at the equator would be shorter as you move towards the poles. For RivEX to process or generate meaningful measures of distance the dataset must be in a projected coordinate system where it's units are linear.
The solution is to project your river network into a coordinate system using linear units, typically metres. You use the Project tool to complete this task. It is strongly recommended that you project your dataset into a File GeoDatabase Feature Class and not as a shapefile.
It's possible that you have been provided a dataset missing it's coordinate system information (a problem often associated with shapefiles and another good reason to avoid this old data format). In this case, if you know what it is, you need to set it using the Define projection tool.
If your data has been provided in WGS84 format, which projection should you use? This depends entirely on where in the world your river network is. Hopefully you will agree you would not project a network into British National Grid if the rivers were located in Australia! Many nations have national grids, you need only search the internet to find out about them, but if you are working in a region of the world where there is no obvious national grid where do you go to determine what to project your data into?
- ESRI provide a Story Map describing the many projections supported by ArcPro.
- ESRI provide a quick notes pdf document that help you narrow down an appropriate projection.
- These three websites will help you identify projections for a country: epsg.io, spatialreference.org and projectionwizard.org
- UTM is a global projection coordinate system that uses metres and divides the globe into zones.
In ArcPro whilst choosing the projection, if you click on Details it displays a dialogue with a map showing a blue rectangle identifying the region of the world that the projection is covering. Use this to verify it is covering the region you intend to project into.
