Spatial Analysis Lab (SAL)

 

Getting WGS84 GPS Data into ArcGIS

 

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See also Adding X-Y Data to ArcGIS

See also How to get data from Excel to ArcGIS

 

By default, GPS receivers record data in latitude-longitude using the WGS84 (World Geodetic System 1984) datum (WGS84 is �an earth fixed global reference frame, including an earth model.� See http://www.wgs84.com/wgs84/wgs84.htm). Many GPS receivers allow the user to view their location in local coordinates as well (e.g., UTM, State Plane, etc.), performing a translation from WGS84 to the desired coordinate system (and datum). Similarly, some GPS software packages allow you to convert to the coordinate system of your choice when you export your data.

        Often, however, GPS data can only be exported in WGS84, typically as a text file with X-Y coordinates along with an ID, a time stamp, altitude and/or other information. Steps for bringing this �raw� GPS data successfully into ArcGIS are outlined below.

 

        Note that this information is somewhat written for our local datasets (i.e., converting WGS84 GPS data to the commonly used coordinate systems in the WWU area: UTM zone 10 North, and State Plane WA North, NAD83 or NAD27).

If you have data that is already in a projected coordinate system (something other than WGS84, e.g. UTM or State Plane) than you can still follow the basic Add XY Data instructions.

 

 

Getting Data From the GPS

        The process of getting your data out of a GPS receiver will vary depending upon the type of receiver and/or the GPS software being used. For the purposes of this tutorial, it is assumed that the GPS data is in a standard ASCII text file, with commas or tabs delimiting the columns of information (not spaces). These columns should include (at the very least) an X (Longitude) and Y (Latitude) field in decimal degrees (if your data is in degrees, minutes and seconds you will need to convert it to decimal degrees). For best results, make sure that the text file:

        Is TAB or COMMA delimited (not space)

        Has only one header row containing the names of the fields

        Uses only standard alpha-numeric characters in the field names

        Has no more than 8 characters and no spaces in the field names

        Has no blank rows

        Has no extra rows of information below the data

        X and Y fields must be numeric


 

Using WGS84 GPS Data in a WGS84 Datum

Open ArcMap

 

Optional: Add Data (from the File menu choose Add Data)

         

Check/Set the Coordinate System and Datum

          From the View menu choose Data Frame Properties

          In the Data Frame Properties window:

                   Click the Coordinate System tab

          Make sure the Current coordinate system is WGS84

          (e.g. GCS_WGS_1984). If not:

                   Use the browse window below Select a coordinate system:

                   to browse to the desired system using a WGS84 datum, e.g.,

                   Predefined | Geographic Coordinate Systems | World | WGS 1984

 

Add GPS data to ArcMap

          From the Tools menu choose Add XY Data�

          In the Add XY Data window:

                   Choose a table� Browse to your data file

                   X Field = <Longitude> (or whatever your E-W field is named)

                   Y Field = <Latitude> (or whatever your N-S field is named)

          For the Spatial Reference of Input Coordinates | Description:

                   (by default this will say �Unknown Coordinate System�)                               

                   Click on Edit (below the Description box)

                   In the Spatial Reference Properties window, press Select

                   In the Browse for Coordinate System window browse to:

                       Geographic Coordinate Systems | World | WGS 1984

                   Press Add

          Back in the Spatial Reference Properties window, press OK

          Back in the Add XY Data window, Press OK

 

Optional: Export data to a WGS84 shapefile:

          Right-click on X-Y data in the Table of Contents and choose Data | Export Data�

          In Export Data window:

                   Check Use the same Coordinate System as the data frame

                   Rename and/or change the storage location of the new shapefile

                       as desired in the Output shapefile box

          Click OK

 


 

Using WGS84 GPS Data w/a NAD83 or NAD27 Datum

          Frequently, GPS data will need to be used in conjunction with local data sets using a NAD83 or NAD27 datum. (NOTE: While the instructions below use ArcMap, ArcToolbox can also be used for defining the projection of your data and/or for re-projection from one datum to another, however, make sure to use the proper Geographic Transformations as discussed above).

            In brief, there are two methods of getting WGS84 GPS data into NAD83 or NAD27 coordinate systems  The first of these is to assign a NAD83 datum for the GPS data (even though the data is actually WGS84) (See discussion on why a NAD83 datum works for WGS84 data below) and provides a simpler, more rapid process for viewing and using the GPS data. The second option requires a few additional steps but should produce slightly better results. Experimental tests indicate that the difference between the two methods to be less than 2 meters. Given the inherent inaccuracy of GPS, the first method may well be �good enough� for most applications. These methods are outlined below, with more complete details on the following pages:

Method 1 � WGS84 to either NAD83 or NAD27

(define the data as NAD83, even though its really WGS84)

  1. Set the Data Frame Coordinate System to use a NAD83 or NAD27 datum, as needed
  2. Add XY Data (as above, setting the Spatial Reference to NAD83)
  3. Export data to shapefile (using the coordinate system of the data frame)

 

Method 2a WGS84 to NAD83

(define the data as WGS84, re-project to NAD83)

  1. Set the Data Frame Coordinate System to a NAD83 datum
  2. Add XY Data (setting the Spatial Reference to WGS84)
  3. Set the Data Frame Transformation for WGS84-NAD83 to NAD_1983_To_WGS_1984_5 (recommended for WA)
  4. Export data to a NAD83 shapefile (using the coordinate system of the data frame)

 

Method 2(b) WGS84 to NAD27

(first get the data to NAD83, then re-project it to NAD27)

  1. Complete steps 1-4 above to get from WGS84 to NAD83
  2. Re-Set the Data Frame Coordinate System to a NAD27 datum
  3. Double check that the Data Frame Transformation for NAD83-NAD27 is NAD_1927_To_NAD_1983_NADCON
  4. Export data to a NAD27 shapefile (using the coordinate system of the data frame)


 

Method 1a Details: Using WGS84 data in a NAD83 Datum

 

Open ArcMap

Optional: Add Data (File | Add Data)

Check/Set the Coordinate System and Datum:

          View | Data Frame Properties

          In the Data Frame Properties window:

                   Click the Coordinate System tab

          If the Current coordinate system isn�t NAD83:

                   In the box below Select a coordinate system: browse

                   to the desired system using a NAD83 datum

                   (e.g., Predefined | Projected Coordinate Systems | UTM |

                   Nad 1983 | NAD 1983 UTM Zone 10N

                        or

                   Predefined | Projected Coordinate Systems | State Plane | 

                   Nad 1983 | NAD 1983 StatePlane Washington North FIPS 4601)

Add GPS data to ArcMap

          From the Tools menu choose Add XY Data�

          In the Add XY Data window:

                   Choose a table� Browse to your data file

                   X Field = <Longitude> (or whatever your E-W field is named)

                   Y Field = <Latitude> (or whatever your N-S field is named)

          For the Spatial Reference of Input Coordinates | Description:

                   (by default this will say �Unknown Coordinate System�)                               

                   Click on Edit (below the Description box)

                   In the Spatial Reference Properties window, press Select

                   In the Browse for Coordinate System window browse to:

                       Geographic Coordinate Systems | North America | North

                       American Datum 1983.prj

                   Press Add

          Back in the Spatial Reference Properties window, press OK

          Back in the Add XY Data window, Press OK

 

Optional: Export data to a NAD83 shapefile (right-click on X-Y data in

                   the Table of Contents and choose Data | Export Data�

          In Export Data window:

                   Check Use the same Coordinate System as the data frame

          Rename and/or change the storage location of the new shapefile

                   as desired in the Output shapefile box

          Click OK


 

 

Method 1b Details: Using WGS84 data in a NAD27 Datum

 

Open ArcMap - Optional: Add Data (File | Add Data)

Check/Set the Coordinate System and Datum

          View | Data Frame Properties

          In the Data Frame Properties window:

                   Click the Coordinate System tab

          If the Current coordinate system isn�t NAD27:

                   In the box below Select a coordinate system: browse

                   to the desired system using a NAD27 datum

                   (e.g., Predefined | Projected Coordinate Systems | UTM |

                   Nad 1983 | NAD 1983 UTM Zone 10N or Predefined |

                   Projected Coordinate Systems | State Plane | Nad 1983 |

                   NAD 1983 StatePlane Washington North FIPS 4601)

Add GPS data to ArcMap

          From the Tools menu choose Add XY Data�

          In the Add XY Data window:

                   Choose a table� Browse to your data file

                   X Field = <Longitude> (or whatever your E-W field is named)

                   Y Field = <Latitude> (or whatever your N-S field is named)

          For the Spatial Reference of Input Coordinates | Description:

                   (by default this will say �Unknown Coordinate System�)                               

                   Click on Edit (below the Description box)

                   In the Spatial Reference Properties window, press Select

                   In the Browse for Coordinate System window browse to:

                       Geographic Coordinate Systems | North America | North

                       American Datum 1983.prj

                   Press Add

          Back in the Spatial Reference Properties window, press OK

          Back in the Add XY Data window, Press OK

 

Optional: Export data to a NAD27 shapefile (right-click data layer in

                   the Table of Contents and choose Data | Export Data�

          In Export Data window:

                   Check Use the same Coordinate System as the data frame

          Rename and/or change the storage location as desired

          Click OK

 


 

Method 2a Details: Using WGS84 data in a NAD83 Datum

Open ArcMap

Optional: Add Data (File | Add Data)

Check/Set the Coordinate System and Datum:

          View | Data Frame Properties

          In the Data Frame Properties window:

                   Click the Coordinate System tab

          If the Current coordinate system isn�t NAD83:

                   In the box below Select a coordinate system: browse

                   to the desired system using a NAD83 datum

                   (e.g., Predefined | Projected Coordinate Systems | UTM |

                   Nad 1983 | NAD 1983 UTM Zone 10N or Predefined |

                   Projected Coordinate Systems | State Plane | Nad 1983 |

                   NAD 1983 StatePlane Washington North FIPS 4601)

Add GPS data to ArcMap

          From the Tools menu choose Add XY Data�

          In the Add XY Data window:

                   Choose a table� Browse to your data file

                   X Field = <Longitude> (or whatever your E-W field is named)

                   Y Field = <Latitude> (or whatever your N-S field is named)

          For the Spatial Reference of Input Coordinates | Description:

                   (by default this will say �Unknown Coordinate System�)                               

                   Click on Edit (below the Description box)

                   In the Spatial Reference Properties window, press Select

                   In the Browse for Coordinate System window browse to:

                      Geographic Coordinate Systems | World | WGS 1984

                   Press Add

          Back in the Spatial Reference Properties window, press OK

          Back in the Add XY Data window, Press OK

 

          Open the View | Data Frame Properties

          In the Data Frame Properties window:

                   Click the Coordinate System tab

          Click on the Transformations� button

          In the Geographic Coordinate System Transformations box:

                   Select GCS_WGS_1984 in the Convert from: box

                   Select NAD_1983_To_WGS_1984_5 in the using: box

                   Click OK to close the Transformations window

          Click OK to close the Data Frame Properties window

 

 

Optional: Export data to a NAD83 shapefile (right-click on X-Y data in

                   the Table of Contents and choose Data | Export Data�

          In Export Data window:

                   Check Use the same Coordinate System as the data frame

          Rename and/or change the storage location of the new shapefile

                   as desired in the Output shapefile box

          Click OK


 

Method 2b Details: Using WGS84 data in a NAD27 Datum

 

Complete the Steps detailed in 2a above (bringing WGS84 GPS data into ArcMap and converting it to NAD83 data, exported to a shapefile).

 

Open the View | Data Frame Properties

          In the Data Frame Properties window

                   Click the Coordinate System tab

                    Use the browse window below Select a coordinate system:

                             to browse to the desired system using a NAD27 datum

                   Click on the Transformations� button

          In the Geographic Coordinate System Transformations box:

                   Select GCS_North_American_1983 in the Convert from: box

                   Select NAD_1927_To_NAD_1983_NADCON under using:

                   Click OK to close the Transformations window

          Click OK to close the Data Frame Properties window

 

Optional: Export data to a NAD27 shapefile (right-click data layer in

                   the Table of Contents and choose Data | Export Data�

          In Export Data window:

                   Check Use the same Coordinate System as the data frame

          Rename and/or change the storage location as desired

          Click OK


 

Why use NAD83 instead of WGS84 when going to a NAD27 datum (since the data really is WGS84)?

 

For our part of the world, WGS84 and NAD83 are very close to being the same. This is why Method 1 above (simply defining your WGS84 data as NAD83) works.

 

However, for ArcMap to use WGS84 data in a NAD27 data frame, a Geographic Transformation must be applied to the data. In the case of WGS84 to NAD27, this involves a mathematical calculation that inevitably introduces a certain amount of error. On the other hand, for Geographic Transformations between NAD83 and NAD27 a much more precise �grid-based� transformation can be used (NADCON: see also http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html). Because the shift between the two datums is not uniform across the entire country there is no single equation that can be applied. Instead, the grid-based transformation uses a series of locally appropriate coordinate calculations (based on a data file which contains the actual datum shift at each point in a network of grid points) to more precisely convert data. Since WGS84 and NAD83 are almost identical, calling your GPS data NAD83 allows ArcMap to use the more accurate grid-based method of transformation, and will typically give better results than transforming directly from WGS84 to NAD27.

 

A more precise method is to define the data as WGS84, then use ArcGIS to convert from WGS84 to NAD83 (using NAD_1983_To_WGS_1984_5 for the Geographic Transformation recommended), and then using the NADCON Geographic Transformation to convert from NAD83 to NAD27. This two-step process takes into account the slight differences between WGS84 and NAD83, while also taking advantage the superior NADCON geographic transformation to convert to NAD27.

 

 

 

 

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