Issue 50

Using GPS Data with Surfer and Grapher

Introduction

Many GPS units can be set to record hike, bike, ski, boat, and car trips to a track. Surfer and Grapher can be used to display tracks as maps and elevation profiles to help you visualize the data.

Maps and profiles created from GPS track data in Surfer and Grapher.

Downloading GPS Tracks to an ASCII File

Surfer 8 and Grapher 5 do not have a way to download data directly from your GPS, but there are many free utilities that can be used for this task. The G7toWin utility downloads tracks and other data from Garmin and other GPS units, and saves the data to an ASCII text file.

Open the track data in the Surfer worksheet to view and delete spurious points.

The following example uses UTM meters for easting and northing, elevation in feet, and distance in miles. Decimal degrees for longitude and latitude can also be used for the X and Y values. West longitudes and South latitudes are negative.

The data start in row 8, with X (easting) in column C, Y (northing) in column D, time in column H, elevation in column J, cumulative distance in column O.

Using Surfer Post and Classed Post Maps to Display Tracks

Since the data are stored as points, the post map is a good first choice to display the track.

Post map of track data shows points,
but lacks other information.

The points are displayed, but it is difficult to get a sense of the start, end, and direction of the track. Since the data include cumulative distance, a classed post map with colors representing the distance displays additional information about the track.

Classed post map of track data shows
accumulated distance with color to
display start, end, and direction.

In this example, purple represents the start of the trip, and red is the end of the trip. The green, yellow, and orange colors display the track at the north end of the map, showing the order of the points.

Overlaying DRG and DEM Data

Overlaying the classed post map on a DRG base displays additional information.

Track data in classed post map overlaid on
DRG topo map.

The DRG maps for many states are available for free or a small fee. The free maps can be difficult to find, but a search engine like Google can be helpful. This map of the Vail Pass quad is from the US Fish and Wildlife Service site at

http://mountain-prairie.fws.gov/gisdata/Colorado/Data_Not_Assoc_With_NWR/DRGs/

Use the TifTfw.bas script to load the DRG with georeferenced coordinates, and theOverlayMapScaleLimits.bas script to overlay the maps while preserving the scale and limits of the selected map.

Another useful map is the 3D surface map overlaid with the previous maps. Though the track includes elevations (which will be illustrated later in this article), the USGS SDTS DEM data is a better way to get a feel for the entire area. This map was created by downloading the free 10m data from the GIS Data Depot, creating the surface map, and overlaying the DRG base map and the classed post map.

Track classed post data overlaid on a DRG TIF base map and
USGS SDTS 10 m DEM.

Classed Post Maps for Elevation Profiles

The classed post map can also be used to display profile information by choosing the cumulative distance as the X value and elevation as the Y value. The X scale is in miles and the Y scale is in feet. Since the X and Y distances vary greatly, turn off the Proportional XY Scaling option to display the profile. The colors of the classed post map points are the same colors displayed on the XY classed post map.

This classed post map displays the elevation profile with the cumulative
distance classed into different color symbols.

Display Track Data with Grapher Plots

Grapher is another good choice for displaying the track data. It has the advantage of connecting the points with a curve, and allows you to fill the area beneath the curve.

Elevation profile displayed as a line graph with fill under the curve in Grapher.

Grapher also has the advantage of reading date and time data. An elevation profile vs. time illustrates long rest periods at the top and bottom of hills as horizontal lines.

Elevations plotted vs. time to highlight stops.

The Grapher XYZ Scatter Plot provides another way to display the track in three dimensions.

The XYZ Scatter Plot provides a way to display 3D data in Grapher

This plot shows the profile in 3D by displaying the northing and easting locations as well as the elevation. To create this plot choose the easting column for X, the elevation for Y, and the northing for Z. Note that the Y and Z directions are defined differently than in Surfer.

Conclusion

Both Surfer and Grapher have a number of useful plot types for displaying three dimensional data. Surfer works well when creating maps, and Grapher is a good choice for 2D and 3D profiles.

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