shrug-l: My data is not displaying in the correct location. What do I do?

[Tripp Corbin]

I have been taking part in a discussion on another GIS list about coordinate
systems, projections and transformations. This all stemmed from a person
that had data which was reported to be in one coordinate system but was not
overlaying other information in the same area and coordinate system. 

 

My experience has shown me this is not an uncommon occurrence. It seems that
many GIS users do not understand how to properly project data from one
coordinate system to another.  I believe this is largely because they work
primarily in a single local area with their own data. It is only now with
the advent of services like ArcGIS Online and Google Earth combined with an
increased need to coordinate activities with multiple agencies that it is
now becoming more common place to share and use data from outside sources
which may be in different coordinate systems and projects. 

 

Because many GIS users have not had experience projecting data from one
coordinate system to another, they choose the wrong tool or method. Instead
of actually projecting the data, they instead just assign it the new or
desired coordinate system using the Define Projection tool or just change
the Coordinate system under the Shapefile or Feature Class properties
believing this projects the data from one coordinate system to another. 

 

Neither of these method actually project data from one coordinate system to
another. All it does is change the coordinate system ArcGIS or other
software packages believes the data is in. Which leads to more headaches and
data displaying in the wrong location. Sadly if a user has done this with
all of their data, they may believe it is correct because it appears to
properly overlay itself. It is only when you try to add data from outside
sources that the problem may become apparent.

 

So what do you do if you get data that has been assigned the wrong
coordinate system because someone used one of the methods mentioned above?
Assuming you are using ArcGIS, here is my method for fixing issues like
this.  There is no real automated way to do this.

 

1.      Delete the projection and coordinate system for the Feature Class or
Shapefile that is questionable

2.      Open a blank map and add the above mentioned data. You will get a
warning about it missing a spatial reference. That is ok. It is what you
should see.

3.      Look at the coordinates you see as you move your mouse in the map
display. See if they look similar to other values for data you work with
that has a properly assigned coordinate system. If they are in the 0,0 or
5000,5000 or 10000,10000 range, then it is possible the data is not
georeferenced at all and will need to be using another process. This often
happens with data that was created by surveyors or engineers and converted
from CAD.

4.      If you identify some existing data that you think may match your
questionable data, add it as a layer. If they align, then you have
identified the correct coordinate system for your questionable data. If not
time to move to trial and error method.

5.      With the data you added above that is in an established and verified
coordinate system, start changing the coordinate system to the data frame
until you get the questionable data and the known data to line up. I would
recommend starting with the following:

a.      Local State Plane NAD 83 (original NAD 83 not HARN, CORS 96, 2011)

b.      Local State Plane NAD 83 variants also try different Units

c.      Local State Plane NAD 27

d.      Local UTM Zone

e.      Geographic WGS 84 (especially if data was captured with GPS)

f.       Web Mercator Auxiliary Sphere

g.      Start running through any other coordinate systems commonly used in
your area

6.      Once you have identified the coordinate system, use the Define
Projection tool to assign the correct coordinate system to the data in
question.

 

If your data is not Georeference or you are unable to determine the actual
coordinate system for the data in question, then you will need to go through
a process to georeference it. The process will depend on the type of data
you have to work with. CAD and Raster data use the Georeferencing toolbar.
Geodatabase Feature Classes and Shapefiles use the Spatial Adjustment
toolbar. I may explain those processes in the future.

 

For those that encounter georeferenced data that has been assigned the wrong
coordinate system, I hope this helps.

 

Tripp Corbin, MCP, CFM, GISP | Chief Executive Officer
 <http://www.egisassociates.com/> eGIS Associates, Inc.

 <mailto:tcorbin at egisassociates.com> tcorbin at egisassociates.com |
www.egisassociates.com 

678-710-9710 ext 21 | 866-304-3864 Fax

Esri Certified Trainer | Esri Certified Desktop Associate | Esri Certified
Enterprise System Design Associate

 

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