Author Archives: mdhyslop_gis

Color selection help

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Color selection for maps can be a time-consuming activity. Computer monitors use additive colors (combinations of red, green, and blue) to create different shades, and monitors are backlit, using transmitted light to convey colors to the viewer. Printed maps, on the other hand, use subtractive colors (most often cyan, magenta, yellow, and black inks) to remove portions of white light reflected back to the viewer.

The main effect of these differing methods for producing colors is that hues you select on the monitor often look very different when printed out in hardcopy. This problem is compounded when different computer monitors or printers are used, as colors you were happy with can change dramatically when viewed or printed on different hardware.

Fortunately, a tool exists to help with choosing a set of colors for a map. ColorBrewer was developed by Cindy Brewer and Mark Harrower of Pennsylvania State University. It is an interactive, web-based tool that allows the user to 1) select the number of color classes needed, and 2) choose color schemes appropriate for numeric data (sequential schemes), numeric data with positive and negative values (diverging schemes with a neutral color in the center and darker colors at the extremes), or nominal data (qualitative schemes).

The color schemes in ColorBrewer were tested with subjects with color-impaired vision. You can choose to use only “color blind safe” colors, as well as “print friendly” and “photocopy-able” colors, though these choices limit the number of color schemes available. The best reason to use ColorBrewer, however, is the colors it provides are easily distinguished from each other.

ColorBrewer provides color information usable in several different coding systems (RGB, CMYK, and Hexadecimal) so it is useful for more than just colors for maps in a GIS. The values for your ColorBrewer color schemes can be exported to an Excel spreadsheet, and there is a plug-in tool for ArcGIS available. It is very user-friendly, but has excellent online help if you need it. Check it out!

Citing geospatial data

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A common question that arises when writing a paper, thesis, or dissertation, is “How do I cite maps or spatial data”? I have cobbled together some examples over the years, found at various locations on the web, into a web page. Some of the source sites are no longer available (except perhaps at the Internet Wayback Machine), but the sources are still listed on the web page below.

A number of examples were drawn from: Cartographic Citations: A Style Guide, MAGERT Circular No. 1. Chicago: American Library Association, 1992. Thanks to the original authors of this publication.

See this page for my list of geospatial citation examples. I will update these examples as time permits.

How to generate random points in ArcGIS

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One common activity in the natural resources field is the assessment of resources, such as vegetation, wildlife, or soil, in an area. There are several strategies available to perform these assessments. One method is to generate a number of random points in a given parcel of land where sampling will occur.

ArcGIS can generate a specified number of randomly-placed points within the boundary of a layer or inside a selected feature in a layer. The tool used to generate random points is found in Arc Toolbox : Data Management Tools > Feature Class > Create Random Points.

A one-page tutorial on generating random points is available here, or see below. It applies to ArcGIS 9.x and 10.

Generating Random Points in ArcGIS
A) Prepare a map document in ArcMap. Add any layers you need, including a polygon layer in which you would like to generate random points (e.g., stand boundaries). If necessary, select the polygon in which you would like the points contained. Verify that the data frame has the correct coordinate system defined.

B) Open ArcToolbox and open the Data Management Tools > Feature Class > Create random points tool.

C) Choose an output directory and a file name (random_points is suggested). If you have a constraining layer (a forest stand boundary or ownership boundary) select it. Enter the number of random points you need. Finally, if you need the points to be a minimum distance apart from one another, enter this value (E) and click OK. The points will be created and added to your map.

D) If you later need to identify your random points (which is likely), open the attribute table,
right-click on the field heading CID and choose Field Calculator. Build an expression
[FID]+1 and click OK. Your points should now be numbered from 1 to N. To add the coordinates for each point to the attribute table, open ArcToolbox > Data Management
Tools > Features > Add XY Coordinates. Choose your random point shapefile and
click OK. The fields POINT_X and POINT_Y will be added to the attribute table.

Data transfer using DNRGarmin

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DNRGarmin is free software produced by the Minnesota DNR. It communicates with Garmin-brand GPS units so you can move point and track history files from your GPS to your computer and back. It has the added benefit or being able to read and write files in several other formats, most notably .kml (Google Earth), .gpx (Garmin), and .shp (ESRI shapefile, usable in ArcGIS).

To use DNRGarmin you will need a data cable for your GPS. Depending on your PC and GPS unit, you may need a USB—serial converter, as newer desktop and laptop PCs do not have serial ports, and older Garmin units only have serial connections available.

A brief, two-page tutorial on using DNRGarmin with ArcGIS is available here. Be aware this tutorial was written for use in the western Upper Peninsula of Michigan. Some modifications will be necessary for other areas of the USA or world.

 

The Michigan GeoRef coordinate system

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Spatial data provided by the State of Michigan is distributed in a Michigan-specific projected coordinate system named Michigan GeoRef. The type of map projection is known as “Hotine Oblique Mercator”. This coordinate system uses NAD1983 as the datum, but the coordinates are unique to—and usable only in—Michigan. It is supported by ESRI software (ArcGIS) as well as DNRGarmin, which can use the ESRI projection information. The POSC code for NAD 1983 Michigan Georef (meters) is 102123.

In ArcGIS (Version 9.2, 9.3, and 10) the projection information (.prj) file can be found in Coordinate Systems : Projected Coordinate Systems : State Systems : NAD 1983 Michigan GeoRef (Meters).prj

In ArcGIS (Pre-version 9.2) the projection information (.prj) file can be found in Coordinate Systems : Projected Coordinate Systems : National Grids : NAD 1983 Michigan GeoRef (Meters).prj

More information on Michigan-specific coordinate systems is available from either Michigan State University or from the Michigan DNR

 

All about this site

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This web site exists to disseminate information about Geospatial resources at Michigan Tech. In the near future it will contain information about software and data resources at Michigan Tech, plus tips and tricks for using geospatial software, as well as news about available training and recent research related to Geographic Information Science (GIS, or Geographic Information Systems, and GPS — Global Positioning Systems).