How to place a surveyed point that has a "Combination Factor" (See attached survey)?

[JamiePetersen]

I have a Public Land Survey Corner Certificate (See attached).  I'd like to place this point into a feature class.  The destination feature class is Iowa State Plane South NAD83 US Ft.  What's throwing me off is that there are two coordinates listed - geodetic & state plane - and the state plane coordinates have a "Combination Factor" listed.  What I would normally do is just set my coordinate system within the data frame to IA State Plane South and then place the point using the Absolute X,Y edit function.  But what do I enter for the coordinates?  The exact Northing and Easting as documented?  Or do I need to apply (multiply or divide?) the given values by the "Combination Factor"?  Something else?  Thanks for any advice in advance.

[TimHodson]

The combination factor does not need to be applied in any way when entering the coordinates, though it is important information to store together with the entered values. The combination factor is scale factor that is computed based on the geographic datum (+ plane projection), the latitude and longitude, and the Elevation of the point. It is applied to "measured" distances for conversion between ground and grid distance values. (Ground distance x factor = Grid distance)

When storing control points, and other land records in a database or GIS, it is important to capture the original information from the document, and to do some cross-checks (sometimes they are just "rough" checks) to, firstly, validate the values on the document, and secondly, to confirm/QA that you've entered the values (projected coordinates in this case) correctly. In this example the check is to confirm that the geographic coordinates are a match with the projected coordinate equivalents. A quick way to do this is to use ArcMap's Identify tool, and change the coordinate units, as described below.

You could store this control point information using a standard point feature class, though I�??d recommend using a parcel fabric, since a control point in the fabric is not too easily (accidentally) moved. In the fabric, a control point can only have its position changed by directly editing/re-typing the control point coordinates in the control point dialog.

Here is a link to a map package that contains the final result with this single control point in it. There is a topographic base-map included in this map that can be used as a check on the Elevation data. Below are some screen-shots of what is represented, and the steps used to create this mini-map.

In these suggested steps below, I make the assumption that you are using ArcGIS Desktop 10.1 and that you are running with an ArcEditor license, or higher. The end result may also be achieved using 10.0, though 10.1 makes some of these steps simpler.

1. Open a new map document in ArcMap
2. In the Catalog window in ArcMap, right-click your geodatabase, and create a feature dataset. Give it the Iowa South NAD83 projection: NAD_1983_StatePlane_Iowa_South_FIPS_1402_Feet
3. Right-click the new feature dataset, and click New > Parcel Fabric
4. Name the fabric, click Next, and accept all the defaults in the Parcel Fabric creation wizard.
5. After you click Finish, the fabric is added to the map.
6. Still in the Catalog window, expand the parcel fabric node, Right-click the fabric class that ends with �??_Control�?� and click properties.
7. Add 2 new fields with the type of [double] and call them EllipsoidHeight, and CombinationFactor.
8. Add another field with the type [text] and call it Description. Add any other fields that you feel are necessary to capture the relevant information from this document. In this case I have used the fields specified in the Local Government Information Model for control points.
[ATTACH=CONFIG]17983[/ATTACH]

9. Right-click the fabric class that ends with �??_Control�?�, click Manage > Create attachments. (this is an optional step, but it can be used to store the scanned document in the database)
10. Turn on the Parcel Editor toolbar, and turn on the Editor toolbar.
11. Click Parcel Editor > Start Editing
12. Click the Maintain Control Points button on the Parcel Editor toolbar.
13. Click the new button, type in the projected coordinates, elevation and other information, click Update, then Close.
[ATTACH=CONFIG]17984[/ATTACH]

14. On the Editor toolbar, click the Attributes button, click Attachments, click Add, then browse to the location of your scanned document to attach it as a reference to this control point. Note, you may attach more than one file as a reference.
[ATTACH=CONFIG]17985[/ATTACH]

15.You can use the Identify tool to see the control point's information.

16.While in the identify window, change the units to degrees, minutes, seconds, then click on the control name node in the tree-view, and check that the coordinates shown match (to 3 decimals of a second) with the latitude and longitude values from the document.
[ATTACH=CONFIG]17986[/ATTACH]

17. Using the public topographic basemap, you can also get a rough check, to the nearest foot, of the Elevation value: in the Identify dialog, change the target to "Elevation (ft)" then click on a location as near as possible to the control point that you entered.
[ATTACH=CONFIG]17982[/ATTACH]

-Tim (Parcel Editor Team)

[JamiePetersen]

Tim - great answer!  I appreciate that.  Actually, the information/steps you provided is right on topic as we're preparing for a parcel fabric migration through the professional services team.  I wanted to have my information straight on our control points before we got started.  Thanks again.

[JamiePetersen]

Just one more follow up question regarding the combination factor: Do I understand you correctly in saying that the combination factor would be used by an individual who was trying to convert lat/long grid coordinates/measurements back to ground coordinates?

[TimHodson]

The grid coordinates are based on grid distances from the point of origin of the plane/grid defined by the projected coordinate system. In this case the lat/lon coordinate was used as a cross-check to confirm that the coordinates on the document are in grid. So to answer your question, yes: to convert the grid coordinates to ground coordinates you�??d apply the Combined Scale factor.

Note that in the fabric (and for most typical projected GIS datasets) the coordinates for control are expected to be in grid coordinates, and the coordinates computed for the parcel points are also in grid coordinates. In the case of the document that you provided, the State Plane coordinates presented are in grid, because otherwise the lat/lon values shown in the Identify window as described in the previous steps, would not match the lat/lon values shown on the document.

Since you�??re planning on working with the fabric, another important related note: all the distance values stored as attributes on the parcel lines are expected to be in ground distances; this is because distances placed on land record documents in the US are ground values. So for example, if you identify a line in the fabric, you�??ll expect to see the line�??s �??shape_length�?� to be different to the value in the distance field. The �??shape_length�?� is the grid distance.

Please also see this archived thread on how the combined factor is used, relevant info quoted here:
�??�?�a particular projected coordinate system (based on a particular geographic coordinate system) results in a plane, or grid. This plane may not align that well when working out in the field due to the average elevation or the distortions due to the projection. The grid coordinate are then scaled to better fit the local area.�?�

-Tim