Monthly Archives: October 2011

DNRGarmin and newer Garmin GPS models

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Newer versions of Garmin GPS units (Colorado, Oregon) do not connect to your computer in the same fashion as the older models (GPSMap 76, GPSMap 62). These newer Garmins mount their internal storage as a drive letter, e.g., “E”, similar to mounting a flash drive or external hard drive.

To download data from one of these units to DNRGarmin, Go to File->Load From->File… Select “GPX” as the file type and navigate to your GPS’s drive (the drive letter that shows up when you plug in your device). You’ll find a folder housing a bunch of GPX files with time-stamp names. Those contain your waypoints. Select one to load it into DNRGarmin and then save out to a new file (text file for backup or shapefile for use with ArcGIS).

To upload from DNRGarmin to the GPS, go to File–>Save To–>File… and save a GPX file to the same location on your GPS as that you downloaded from above.

Transferring track history points should work in the ‘normal’ DNRGarmin fashion.

Note I do not have access to one of these newer units – these instructions are written from “help me” postings I have found online. If you have an Oregon or Colorado unit and would like help using DNRGarmin, I will be happy to do so (and can then verify these instructions).

mdhyslop at mtu.edu or 7-2308

Store relative pathnames for layers in ArcGIS

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By default in ArcGIS 10, full pathnames are stored for all layers you add to a map. This means if you have a file located at c:\data\hydro\rivers.shp that’s used in a map, the full pathname (c:\data\hydro\rivers.shp) to the layer is stored in the map document. ArcMap will always look in c:\data\hydro for the file rivers.shp. Using relative pathnames allows you to move map documents and layers around, as long as the relative locations of the map document and the data don’t change, e.g., as long as the map.mxd map document and the folder data (containing a subfolder hydro with the file rivers.shp, shown below) are moved together, ArcMap will still be able to open the map without issues.

To make the change to relative paths for a single map, choose Map Document Properties… from the File menu and check Store relative pathnames to data sources.

If you want all your new maps to be stored with relative paths you will need to specify relative paths as the default. To do so, click Customize > ArcMap Options… to open the ArcMap Options dialog box, and then click the General tab. Check the option to Make relative paths the default for new map documents. This setting is stored in the registry.

Why should you care about these settings? If you move map documents and data around, say on a flash drive or portable hard drive, these devices can be assigned different drive letters (e, f, g, i…) depending on what computer they are plugged into. If the drive letter changes between sessions and you are using full path names, ArcMap will be unable to find some, or all, layers in your map, and you will be presented with an exclamation point next to the ‘missing’ layers:

 Speaking from personal experience, it is a pain to have to re-connect more than a few missing layers, and some maps have dozens. What’s the process to fix a missing layer?

  • Open the layer’s properties, either by double-clicking on the layer name or right-clicking on the layer and choosing Properties…
  • Click the Source tab and then the Set Data Source… button, browse to the layer’s location on your computer, select it, click add, then OK.
  • Repeat for each missing layer.

To avoid having to repair any map documents in this fashion, you should either a) never move your map documents and associated files around, or b) use relative pathnames, and store your map documents and data in the same directory (or subdirectories).

Google Earth blog

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A great source to find news, new data, and interesting features to explore in Google Earth is the Google Earth blog. If you’re more interested in “odd” things found in GE, try map of strange (not affiliated with Google).

While I’m on the topic of odd, check out Strange Maps, a blog showcasing some unusual print maps, including “US States renamed for countries with similar GDPs“. Those who aren’t used to the ‘rustic’ nature of much of the Upper Peninsula might enjoy the “Map of the area surrounding our holiday home“.

Using GIS to determine stream sinuosity

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Sinuosity is a measure of how much a river (or other linear feature) deviates from being straight. A truly straight river or road has a sinuosity of 1; as the number of meanders increases, sinuosity approaches 0.

I wrote a document describing how to determine the sinuosity of linear features years ago. I will update it for ArcGIS 10 as time permits, but the original document, written for ArcView 3.x and ArcGIS 8x/9x is available here.

Importing x,y data into ArcGIS

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Importing a set of x,y coordinate pairs into ArcGIS is very straightforward, and can be a useful skill if you need to bring in simple coordinate data that is not already in a spatial format.

1) Acquire some data you want to display on a map. The source doesn’t matter: it might be locations recorded on a GPS unit that you don’t have a data cable for, coordinates determined from a topographic map, or a simple data set found on the internet. If you want your locations to integrate correctly with other layers (always a good thing in GIS) you will need to know both the coordinate system and datum used by the source data, e.g. latitude / longitude and NAD27.

2) Format your data properly. This may be done in a spreadsheet program such as Google Spreadsheets <spreadsheets.google.com>, or the more ubiquitous Excel. To import properly into ArcGIS, your data file will require a minimum of three fields: a unique identifier for each data point, and the X (easting) and Y (northing) for each location. Other attribute data, if available, may go into additional columns, but your points will import fine with just these three pieces of information. Your file should also contain a header line that describes each field. It might be as simple as ID, X_coord, Y_coord. Additional fields are OK, but descriptions in each header are limited to seven characters and cannot have special characters or spaces. If you need spacing for readability, use underscores.

Be sure if you are using latitude/longitude coordinates that your northings are positive if north of the equator and eastings are negative if you are in the western hemisphere. Example: the coordinates for a location in Houghton, Michigan are -88.54820, 47.11535 in latitude/longitude and 382552, 5219145 in UTM zone 16, NAD 1983 datum. Omitting the “-” sign will put your points on the wrong side of the globe.

If your coordinates are in latitude/longitude, they should be in decimal degrees (DD) format before importing into ArcGIS. Locations in degrees, minutes and seconds (DMS) or decimal minutes (DM) format should first be converted to DD. There are converters available on the internet, but it is probably quicker to use Excel to perform the conversion.

D = Degrees  M = Minutes  S = Seconds  .m = Decimal Minutes  .s = Decimal Seconds

DM.m = Degrees, Minutes, Decimal Minutes (eg. 45°22.6333)
D.d = Degrees, Decimal Degrees (eg. 45.3772°)
DMS = Degrees, Minutes, Seconds (eg. 45°22’38”)

DMS –> DM.m (45°22’38” –> 45°22.6333)
◦ Divide S by 60 to get .m (38/60=.6333)
◦ Add .m to M to get M.m (22+.6333=22.6333)

DM.m –> D.d (45° 22.6333 –> 45.3772)
◦ Divide M.m by 60 to get .d (22.6333/60=.3772)
◦ Add .d to D to get D.d (45+.3772=45.3772)

D.d –> DM.m (45.3772 –> 45°22.6333
◦ Multiply .d by 60 to get M.m (.3772*60=22.6333)

DM.m –> DMS (45°22.6333 –> 45°22’38”)
◦ Multiply .m by 60 to get S(.6333*60=38)

DMS–>DD
◦ D + M/60 + S/3600 = DD

3) Save/export your data as a comma-delimited text file (CSV format in Google Spreadsheets or Excel). You may want to save a copy in native format (.xls in both Google spreadsheets and Excel) before exporting to .csv.

4) Add your .csv file to ArcMap by using the Add data tool (either by expanding the submenu under File> Add Data)   or by clicking the Add Data tool on the Standard toolbar 

5) Right-click on your new layer and choose Display XY Data… Ensure that the X and Y fields were selected correctly by ArcMap – they should be right if you chose names that reflect the coordinate positions (northing and easting or x and y). Click the Edit… button, then Select… to select the coordinate system for your points, Add… and OK (3x). The correct coordinate system to use may be obtained from your GPS unit (under map setup or units); from the marginal information on your topo map; or from the metadata file that came with internet data. Look in geographic coordinate systems for latitude/longitude projection files, or in projected coordinate systems > UTM for UTM data.

Some possible suggestions:

  • Coordinate Systems > Geographic Coordinate Systems > World > WGS1984.prj  (default GPS datum)
  • Coordinate Systems > Geographic Coordinate Systems > North America > NAD1983.prj  (current North American datum)
  • Coordinate Systems > Projected Coordinate Systems > UTM > NAD1983 > NAD 1983 UTM Zone 16N.prj  (appropriate for most of the U.P.)
  • Coordinate Systems > Projected Coordinate Systems > State Systems > NAD 1983 Michigan GeoRef (Meters).prj  (projection and datum used by the State of Michigan)

6) You should now have a point layer at the top of your Table of Contents with the same name as your .csv file and the word Events on the end of the name.

event layer

This is an “event theme” and is a temporary layer. If you want a more permanent copy, right-click on the layer and choose Data > Export Data… Pick an output location—a geodatabase feature class or a directory for a shapefile—and enter a file name. Please change the name to something other than the default Export_Output – I suggest one that more accurately reflects the contents of the data layer. Click OK.

You now have a permanent layer, either a geodatabase feature class or shapefile, of your original ASCII (text) coordinates.

Geocoding in ArcGIS Online

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Geocoding is the process of interpolating a location from an address. Data used in geocoding does not have a spatial location (coordinates) and is usually stored in a database or spreadsheet.

ESRI has developed a tutorial for ArcGIS Online that shows how to perform geocoding. It is available here.