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In ArcGIS 10.1 (advanced lisence) I have imported the plan of a site. The plan has no coordinates, and I couldn't georeference the plan since it has undergone changes during the last 30 years. So, the map has no spatial reference. I used polygons to vectorialize some features I am interested in. Since my ultimate goal is to perform some spatial statistic analysis (e.g., calculating Moran's I for a numeric attribute stored for each feature), how can I devise an arbitrary coordinate system for the site in order to be able to use distance-based statistics?

As for earlier similar question: a somewhat similar question here (projection and coordinate system for non georeferenced archaeological site) got no replies. Further, another often-quoted thread of this forum (Using customized Coordinate System in ArcGIS Desktop?) does not address my issue. At the best of my understanding, what is suggested there entails that "...You have to know the geographic coordinates lat_0 and lon_0 of the origin of your CRS". This does not seem to apply to my issue since I do not know the coordinates of the origin of the coordinate system.

NewAtGis
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  • Does it have a scale? – FelixIP Sep 22 '17 at 18:00
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    Are the internal coordinates consistent? That is, if you measure from point A to point B, does the distance match the ground distance? If so, then I might try to write up an answer. If you follow the "this is a duplicate of" link in your link, that would be a similar answer. – mkennedy Sep 22 '17 at 18:29
  • If I have well understood your comment, I think the answer is yes. The plan is an area in which during the 1960s they have unearthed a cemetery. The site plan has (needless to say) a scale, the distances between the graves are (obviously) measurable on the plan. But, once I imported it in GIS and vectorialised the graves using polygons, the resulting vector layer (as well as the improted map) has no coordinate system. I hope I addressed your questions. Thanks. – NewAtGis Sep 22 '17 at 18:32
  • @DanC: I do not believe my question is a duplicate of that post. At the best of my understanding, the answer provided there entails knowing "...the geographic coordinates lat_0 and lon_0 of the origin of your CRS", which does not appy to my case. – NewAtGis Sep 22 '17 at 21:22
  • You don't need to know the exact coordinates of your map's origin point, because all you care about here is that the scale is correct, not that everything is in its correct real-world position. So you can just go to the general area where your cemetery is and pick a spot for your origin. – Dan C Sep 23 '17 at 12:27
  • @DanC: thank you. I would be thankful if you could guide me through the required steps to accomplish that in ArcMap. Cheers – NewAtGis Sep 23 '17 at 12:59

1 Answers1

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Because you're interested in local and relative distance-based analysis, you should be able to use a fake coordinate system.

ArcGIS has a "Local" projection. It's like Orthographic but is for much smaller areas, treats the Earth as an ellipsoid, and treats the area of interest as a flat plane.

If you wanted to connect the site plan to other GIS, georeferenced data, you would need to do more work--identify the geodetic coordinates of at least one point, preferably more so the site plan could be georeferenced, or in ArcGIS terminology, spatially adjusted to a known coordinate reference system.

If the digitized/vectorized features already scaled to return ground distances, custom a "Local" projected coordinate system by setting whatever you want for the longitude and latitude of center. You could give some coordinates are near to the site plan's geodetic location, but that might backfire in the future because someone might think the location is correct. So feel free to use (0,0).

If the point you want for the center of the projection (center of site plan) doesn't have 0,0 for its projected coordinates, add those values to the false easting and false northing parameters.

Leave the scale factor set to 1.0. The azimuth parameter will rotate the site plane relative to geodetic north. I would start with this left equal to zero too. Set the units to meters or whatever unit the ground distances are in.

For the geographic coordinate system, it shouldn't matter, you might as well use WGS84.

Use the Define Projection tool to assign this coordinate system to the data. You can create the custom PCS while running the Define Projection Tool.

mkennedy
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  • thank you for the detailed reply. Please, bear with me since I am not that familiar with coordinate systems. When you talk about "local" projection, are you referring to this new facility: http://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/local-cartesian-projection.htm. If you do, it seems to be available from 10.3 onwards. I am actually using 10.1. If you are not referring to that, I understand that I have to customize an already implemented (projected) coordinate system. If this may help, I know that the site falls somewhere within the UTM Zone16N. – NewAtGis Sep 23 '17 at 06:58
  • Should I be supposed to customize the UTM Zone16, I understand (but correcting me if I got it wrong) that I should set 0 both both the 'false easting' and 'false northing'. But, which value shall I insert for 'central meridian' and 'latitude of origin' parameters? – NewAtGis Sep 23 '17 at 07:04