Using Spatial Adjustment to align Arbitrary Coordinates to Real-World Coordinate
The common practice of using arbitrary/false coordinates to map archaeological sites makes recording a site on the ground much easier. Also, the use of such arbitrary coordinate systems means that archaeologists typically do not have to do much 'hard' math to figure out distrotions in the Earth's surface and so forth. However, using these non-referenced points makes it very difficult to compare our site data with other features on the planet's surface. Luckily, ESRI's ArcGIS comes with some nice tools to help us shift these arbitrary coordinates into real-world coordinates with just a few clicks.
This tutorial assumes a modest amount of familiarity with ArcGIS. If you've completed the low level tutorials on my website, you should be fine with this one.
The general process is as follows: 1) import your shapefile with arbitrary coordinates into ArcMap, 2) import your shapefile with real-world coordinates - this is typically GPS readings of a few of your arbitrary coordinates (e.g., datum points), 3) use the spatial adjustment toolbar to push/pull the arbitrary points into the real-world. That's it! Of course, as with everything, you'll probably want to play around with it a bit, but it can be really that quick.
Programs used in this tutorial:
Extensions used in this tutorial:
The first thing you need to do is make sure the spatial adjustment toolbar is activated. In ArcMap, click on the "Editor" drop-down menu, go to "More Editing Tools", and click on "Spatial Adjustment".
This will bring up the Spatial Adjustment Toolbar
I am using a cloud of points collected in 2006 at Kingsley Plantation. I have selected the datum points and they show up as highlighted in the image below.
You can see that the two sets of datums look pretty similar. Of course, there will most likely be some difference since most of us use magnetic north to set up arbitrary grids while the GPS most likely uses true north to orient its points. While this is not a serious issue, just be aware that due to different declinations, your arbitrary and real-world points may appear quite differnt.
TIP: Now that you have zoomed to the extents of both your point cloud and your gps points, you can use the "Go Back to Previous Extent" and "Go To Next Extent" buttons during the next step(s). They are located next to the "Full Extent" botton (the little globe).
Now, set the Adjustment Method, click on the Spatial Adjustment toolbar like above and highlight "Adjustment Methods", and select your preferred method. There are five different types of adjustment:
Transformation - Affine = An affine transformation can differentially scale the data, skew it, rotate it, and translate it, affine transformation requires a minimum of three displacement links. They are often used to convert data from unknown digitizer or scanner units to real-world coordinates. (that sounds like what we want, but lets take a look at the other types just to be sure)
Transformation - Similarity = The similarity transformation scales, rotates, and translates the data. It will not independently scale the axes, nor will it introduce any skew. It maintains the aspect ratio of the features transformed, which is important if you want to maintain the relative shape of features, similarity transformation requires a minimum of two displacement links. However, three or more links are needed to produce a root mean square (RMS) error.
Transformation - Projective = The projective transformation is based on a more complex formula that requires a minimum of four displacement links. This method is used to transform data captured directly from aerial photography.
Transformation - Rubbersheeting = During rubbersheeting, the surface is literally stretched, moving features using a piecewise transformation that preserves straight lines.
Transformation - Edgematching = The edgematching process aligns features along the edge of one layer to features of an adjoining layer. It is mainly used when you want to merge separate adjacent layers such as soils or contours sheets etc and you need to ensure the features from those layers will meet at the join.
We will use Transformation - Affine. Select it in the "Spatial Adjustment" toolbar under "Adjustment Methods".
We are almost ready to set the transformation, but first we need to make sure we are selecting the precise points in space. Go to "Editor" click on "Snapping" and click 'vertex' for both the arbitrary and real-world coordinates. Then, click on the "New Displacement Links" button on the "Spatial Adjustment" toolbar. You can close the snapping window by the table of contents by clicking the small 'x'.
No, all you need to do is click on a datum in the arbitrary coordinate point cloud, this sets up the first half of a displacement link. (I've zoomed in and turned on labels)
No, zoom to the real-world coordinates and click on the same datum point (but with real-world coordinates). This creates a link between the arbitrary and real-world point. Now, repeat this for at least three or more points. Remember, always click on the arbitrary point first, then on the real-world point, this way ArcMap will know which way to adjust the point.
Now, click on the "Spatial Adjustment" button and select "Adjust"
and you're done! That's all there is to it! Your points will shift out of arbitrary coordinates and into real-world coordinates.
Still need help? See ESRI's overview of spatial adjustment in the ArcGIS 9.3 online help.