0

I've got attributes for a length that the Near tool calculated, and it gave them to me in decimal degrees. I need them in feet. Is it possible to do this with the Calculate Field tool? I tried reading the documentation in the ArcMap help, but it was really unhelpful. I'm willing to do it in python or VBScript or whatever it takes.

BlakeG
  • 427
  • 2
  • 6
  • 13
  • 4
    I think the easiest thing to do would be to reproject your data and rerun the near tool so you can get back some useful units. – crmackey Jun 03 '15 at 16:51
  • 4
    It is never possible to recover linear units from Cartesian distance of angular units. – Vince Jun 03 '15 at 16:57
  • But there do exist tools which can return great circle distances or even distance along ellipsoid. "Distance" function in SpatiaLite is one such https://www.gaia-gis.it/gaia-sins/spatialite-sql-latest.html. Naturally it needs start and end points in lat/lon as source data. – user30184 Jun 03 '15 at 17:28
  • 1
    Related: http://gis.stackexchange.com/questions/8650/how-to-measure-the-accuracy-of-latitude-and-longitude/8674#8674 and http://gis.stackexchange.com/questions/75528/ – Chris W Jun 03 '15 at 17:35
  • Blake, you mention in comments to an answer that you're having problem reprojecting the data. Are you using the Define Projection tool or the data's property page or the Project Tool. You have to use the latter, which will create a new feature class/shapefile. – mkennedy Jun 03 '15 at 17:36
  • I used the Project tool. It was giving me a totally incorrect output. Looking at the whole dataset, the scale was about 1:500,000,000,000. I ended up exporting the data and checking the option to use the same projection as the data frame, and was able to get a usable result. – BlakeG Jun 03 '15 at 17:59
  • Weird. Thanks for letting me (and everyone else) know. – mkennedy Jun 03 '15 at 20:31
  • What coordinate system are you projecting from and to? Is it possible it's already been set to a projected coordinate system (by accident perhaps)... the only way you're going to get this to work is to project your data to a suitable spatial reference (state plane is ok, but how about UTM (WGS84 UTM Zone 11 North I think would suit). Both data sets need to be in the same spatial reference; it is trivial converting metres->feet, feet->metres so using a metre based spatial reference should do the job. – Michael Stimson Jun 03 '15 at 21:45
  • It wasn't projected. It was in NAD 1983 (2011), which is a GCS. I was just trying to project it to NAD 1983 State Plane California VI FIPS 0406. The Project tool was just giving me all kinds of problems. I just did the workaround I mentioned above to get the correct result. – BlakeG Jun 04 '15 at 16:17
  • 1
    I suspect the problems you were running into with the Project tool (if the data was loaded in to the correct place relative to other data, as your comment about using the dataframe for export seems to indicate) is related to the choice of transformation or lack thereof. 2011 isn't just regular NAD83, so it's actually a different datum than the NAD83 of the state plane projection (unless you're using the updated datum for both sides). Without more specifics on your data and the Project settings you used, it's hard to say. Sounds like you got what you needed though. – Chris W Jun 04 '15 at 18:21

2 Answers2

3

The near tool gives you the results in the linear unit of measurement of the layer's projection. Try choosing a different (appropriate for your dataset) projection that uses feet. Then re-run the near tool.

MWrenn
  • 1,508
  • 8
  • 13
  • I've tried to reproject the data a number of times (going into a state plane system) but it's giving me all kinds of trouble. Ir's not reprojecting correctly so that's why I'm just trying to convert these values. – BlakeG Jun 03 '15 at 16:54
  • 4
    I would fix that problem first. Your distances are going to be more accurate if you're in the appropriate coordinate system. Trouble reprojecting your data may also be symptomatic of other issues. – MWrenn Jun 03 '15 at 16:57
  • I concur. Since a degree isn't a constant unit of measurement (deg of longitude is 69 miles at equator and 0 at the poles) conversion is a pain. Better to use a system that's in feet or meters from the get-go. – fdonnelly Jun 03 '15 at 16:58
  • Okay let's just say I'm not looking for accuracy. All my data points are in San Diego, so the variance in degree lengths is negligible. Is is at all possible to convert these values? – BlakeG Jun 03 '15 at 17:01
  • I've never tried this, but you might see this link - http://gis.stackexchange.com/questions/14449/java-vividsolutions-jts-wgs-84-distance-to-meters Still, I think your best bet is to create another post detailing your projection issues (if you haven't already) and get some help resolving that. Feel free to link it here, and I will take a look at it. – MWrenn Jun 03 '15 at 17:08
  • 2
    Remember that the length of degree is different from West to East and from South to North. Simple multiplication is not enough but you must know the angle of the distance as well. Gets complicated and I suggest to use a better method. – user30184 Jun 03 '15 at 17:33
0

If you need a general near distance parameter in feet you can use the following calculation to convert decimal degrees to feet.

Add a field - right-click for field calculator and multiply the decimal degrees field by 364567. It won't be exact but fairly close.

Information was obtained from a nautical calculation website & verified with the measurement tool.

Yogesh Chavan
  • 2,705
  • 2
  • 9
  • 24
Cödingers Cat
  • 49
  • 3