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An Introduction to Locating Sample Points

Why do you need location data in the first place?

Location data is useful in scientific studies because it helps…

  1. Locate study sites
  2. Locate invasive species populations, Read More
  3. Provide accurate geographic coordinates, allowing others to analyze the data from a spatial perspective.
  4. To put the locations in a form that is easily used by anyone around the world.

It is important to have accurate and precise location coordinates as the data you collect allows scientists to monitor the spread of an invasive species. Scientists make predictions based on location observations. If we know the location of an invasive population (or absence of a population) we can act accordingly, by monitoring or mitigating the further spread of invasive into previously uninhabited areas. Your observations can help us determine where invasive species might be based on where we know they currently are located. It may seem trivial but location data is very important! If location data is inaccurate, scientists can’t use your data in research or land management decisions or to establish regulations to protect the environment from a spreading invasive. Therefore, accurate location data can help make sure the detection and rapid response to invasive threats is as efficient as possible.

As a citizen, you can use location data as a tool to notify land managers of the location of a new invader or to record the locations of a control or monitoring effort.

How does locating a point on the landscape work?

There are a wide range of hand held GPS units and online tools you can use to determine your location on Earth using a “coordinate system”. If you know a little background information on what various “coordinates” mean and how they are derived, they can easily be transferred to specific map locations making your data useful to anyone that you share it with. There are various methods used to indicate study site location, but we will describe how to determine a location using either…

  1. making 'waypoint' using a hand held Global Positioning Systems unit (GPS),or
  2. the online mapping website iTouchMap. There are MANY other online mapping tools but we have chosen to provide instruction on using iTouchMap because it is easy to find the latitude and longitude in the format we request (decimal degrees). For a tutorial on using iTouchMap, click here.

Coordinate Units and Notation (how we write them!)

There are different formats of writing longitude and latitude coordinates, for consistency we use the Decimal Degree format (i.e. 44.98127 latitude, -93.34479 longitude). The more numbers reported after the decimal point, the better your accuracy (which is why the unit is called Decimal Degrees). We ask for at least 5 decimal places to give a reading with at least 50-100 meter accuracy. By convention, the latitude value is listed first, then the longitude value, separated by a comma. In North America, the latitude is represented by a positive number and the longitude by a negative number. To read more about the coordinate system, click here.

There are several other commonly used, and similar looking, formats like Degree Decimal-Minutes (i.e. 64° 68.445 latitude, 92° 42.367 longitude),  and Degree, Minutes and Seconds (i.e. 44° 40' 16.75 latitude, 93° 37' 10.05 longitude).  Degrees Decimal-Seconds and Degrees, Minutes, Seconds are just different formats of reporting a coordinate measurement, like measuring distances in meters versus feet. Similar to the different units of measurement, coordinate types may look similar (all numerical) but are actually very different. Just as 5 feet is very different to 5 meters, a coordinate given in Decimal Degrees-Seconds is very different than a coordinate in Decimal Degrees. To read more about the coordinate system, click here.

It is very important when you report your data that you provide it in the correct format!

For instructions on how to set the coordinate system in your GPS unit to Decimal Degrees, click here. For a copy of the Garmin76 GPS users manual, click here.

Using iTouchMap, you can either locate your point on their map to get latitude and longitude in Decimal Degrees OR convert Degree, Minutes and Seconds coordinates to Decimal Degrees. For a tutorial on using iTouchMap, click here.

What Is A “Datum” And Why Do We Care!?

The Earth is not a prefect sphere, and it’s bumpy! So a coordinate system based on a perfect sphere can lead to big inaccuracies in location data. There is no one coordinate system that fits all the world so there have been many developed to deal with the specific contours of different parts of the globe, called “datums”.

In North America, there are several that are commonly used in different locations. Three common ones are NAD83 and NAD27 (North American Datum 83 and 27, respectively) and WGS_84 (Wisconsin Geographic System 84). As with the format of the coordinates, you can set the appropriate ‘datum’ to use in your handheld GPS unit. When you report your location data be sure to indicate the datum you are using, any of the three NAD83, NAD27 or WGS_84 are fine (though we prefer NAD83). Different datums put the same coordinate values in different physical places on the ground (or on a map, called projections), so it is important that you use, or report, the correct datum!

For instructions on how to view or set the ‘datum’ in your GPS unit, click here. For a copy of the Garmin76 GPS users manual, click here.

iTouchMap uses the datum WGS_84 for their latitude and longitude coordinates.

Additional Resources

If you would like to learn more about your GPS technology or how to better navigate round your GPS unit there are many resources available to you, check out the tutorials at, that were developed specifically for citizen scientists.

If you are a teacher who is going to be developing a class on this topic check out:

the National Institute of Invasive Species Science (NIISS) presentations on coordinate systems and using GPS. You can use or modify them to fit you class’s skills base. Also, the Minnesota Logger Education Program (MLEP) GPS Guide. It is very detailed.

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