point distance/near analysis units?

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6
04-08-2013 11:57 AM
ChrisJ
by
New Contributor III
hi all, have been learning lots here, will have more Qs for sure too...

I'm running both point_distance and generate_near_table on a couple of point layers.  I often specify a search radius of say 1 or 3km, which is easily set, and sometimes I don't set a search radius and take everything back I can get.  either way, the distance columns in the resultant dbfs are in what I think decimal degrees distance; how can I get back a km distance value?  I don't see an option in either dialogue for specifying units, but, I'm a bit new to this.

thanks very much,

cj
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6 Replies
JoelCalhoun
New Contributor III
This is and excerpt from the help for Generate Near Table:

�?�The distances calculated by this tool are in the unit of the coordinate system of the input features. If your input is in a geographic coordinate system and you want output distances to be measured in a linear unit (as opposed to decimal degrees), you must first project your input to a projected coordinate system using the Project tool. For best results, use an equidistant projection or a projection intended for your study area (UTM, for example).
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ChrisJ
by
New Contributor III
thanks for the info stringtheory, I see what you mean.  now, I'm new to arc9.3; if I want a map of all of canada or the US to suit this linear unit measurement I'm after, whereabouts would I change it: right-click 'layers' > properties somewhere?  or need I go into the properties for each individual layer for my .mxd and change it there?  if it's the latter, I only see a 'set data source' under the projection information (to change the source file for a given layer), but nothing that says 'change projection' or anything?  obviously I'm a complete novice here, any layman's terms are appreciated...

thanks all,

cj
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EricRice
Esri Regular Contributor
Chris,

The answer is in the excerpt from above.  i.e. "you must first project your input to a projected coordinate system using the Project tool."  So, you don't really change anything on your data.  You input it into the Project tool and create a new output that uses meters for its linear unit.  Once you get the distances in meters, you can easily add another field and using the field calculator, convert the meters into kilometers.

Best,
Eric
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ChrisJ
by
New Contributor III
thanks for the info eric.  I can't emphasize enough how much of a novice I am with arc, after having come from a long mapinfo background, so if anyone's up for it I could still use more clarification....

- my two point layers, when put into a 'generate near table' are coming back with what I presume are decimal degrees for the DISTANCE column; I would prefer metres.
- point layer 1 ('stores'), under its source is set to geographic coordinate system: GCS_NAD_1983_NSRS2007 and datum: D_NAD_1983_NSRS2007.  it was generated using lat and long columns in the original table to 'add XY data'.
- point layer 2 ('competitors'), under its source is set to geographic coordinate system: GCS_NAD_1983_NSRS2007 and datum: D_NAD_1983_NSRS2007. it was also generated using lat and long columns from the original table to 'add XY data'.
- if I right-click the 'layers' at the very top of the ToC, and go to the coordinate system tab, it shows:

WGS_1984_UTM_Zone_17N
Projection: Transverse_Mercator
False_Easting: 500000.000000
False_Northing: 0.000000
Central_Meridian: -81.000000
Scale_Factor: 0.999600
Latitude_Of_Origin: 0.000000
Linear Unit: Meter

so, I see a mismatch here in that the layers themselves are geographic (GCS_NAD_1983_NSRS2007?) but the 'layers' shows projected (UTM zone 17N)?  is one taking precedence over the other, resulting in a decimal degrees measurement for DISTANCE in the near table?  is there some blatant mismatch here that I should address first, and how?  if something such as the project tool in the toolbox is what I'll need, some step by step instructions would be very handy.  my goal is to have these two point layers on a properly projected map of all of canada (not just one UTM zone), which comes back with metres when I generate a near table...

thanks very much all,

cj
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EricRice
Esri Regular Contributor
CJ,

ArcMap's dataframe (right click 'layers' > properties in the TOC)  does not have to be the same coordinate system as the data in the map (the two point layers).  When you add GCS points to a map defined in UTM space the application projects the data on the fly to UTM.  This is very common in ArcGIS. A single UTM zone isn't suitable for data that covers all of Canada so I'm not sure why the dataframe is set to that rather than something more continental.

For your analysis though, the dataframe coordinate system is irrelevant.  Your data is GCS and that is what the tools are going to use.  So, you must run the Project tool.  The tools is pretty basic.  Give it your input, select an output coordinate system suitable for Canada and distance analysis, define the output name/location, and set any geographic transformations if you're going to be switching datums.  Transformations are only optional if you're not switching datums.


Eric
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ChrisJ
by
New Contributor III
thanks for your help eric; after a bit of clicking I've gotten the measurements the way I want now...

thanks again,

cj
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