The point distance is being calculated improperly between two points.

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08-20-2014 10:34 AM
KangaRoo
New Contributor

I am importing two excel files into ArcMap that contain the coordinates for points in the decimal degree format.

 

The XY data displays successfully and the points are shown on the map, seemingly in the proper positions relative to each other. But when I try to calculate the distance using either the Near Table tool or the Measure tool, what should be a distance of over 1000 meters is calculated to be a fraction of one meter. What can be causing the problem?

 

I suspect it may be the coordinate system. Should I be selecting a specific coordinate system after the XY points are plotted? I am selecting the WGS 1984 by right clicking the point in the Table of Contents on the left, and editing the Data Source to show the Geographic Coordinate as WGS 1984.

 

What is the proper way to find the distance between the points in the attached excel files?

Any help or corrections would be appreciated!

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RobertScheitlin__GISP
MVP Esteemed Contributor

Kanga Roo,

  If you are going to measure distance then you need to set your maps data frame to some projection of your choice that uses meters, maybe UTM 1983 17N, seems how you are measuring on the island of Bimini. Doing that I get something more like 1650 Meters.

Also if trying to use the Near Table tool choose geodesic instead of planar.

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3 Replies
RobertScheitlin__GISP
MVP Esteemed Contributor

Kanga Roo,

  If you are going to measure distance then you need to set your maps data frame to some projection of your choice that uses meters, maybe UTM 1983 17N, seems how you are measuring on the island of Bimini. Doing that I get something more like 1650 Meters.

Also if trying to use the Near Table tool choose geodesic instead of planar.

KangaRoo
New Contributor

Thank you sir!

What projection would I use if I wanted to measure the distance in miles?

The reason I chose WGS 1984 is because I believe it is used by Google Earth.

I guess I would only need to use WGS 1984 if I wanted to export a kmz to be viewed in GE.

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DarrenWiens2
MVP Honored Contributor

You should choose a projection based on: 1.) your study area location, and 2.) what you are interested in measuring (distances, areas, or angles). Projections either preserve distance, area, or angle (or compromise to lessen all three distortions, but fully preserve none), so choose a projection that preserves what you are interested in, in your case distance. The smaller your study area is, the less dramatic the effect of distortion, so in your case you may never notice the effect of choosing an inappropriate projection. The farther your study area is from the projection's "sweet-spot," the more dramatic the effect on your measurements will be, so choose a projection suitable for Bimini, like UTM zone 17 as Robert suggests.

As for units, you can directly calculate miles from meters (meters * 0.000621371). You were seeing widely incorrect numbers using WGS84 for distances because it uses degrees.

If you're interested, you can go here to see the effect of trying to measure distances on an unprojected surface. All circles are 500km in radius.