Lab 5

Jump to Defining projections – Projecting to UTM zone 10 – Projection to CA State Plane III, NAD 83 – Differences between projections

Defining projections

As a first step, I went and defined projections for the faults, census blocks, census groups, and hypsography shapefiles. Here are some screenshots:

Definining a projection

In this window I'm setting the projection for the census blocks shapefile. After these changes, we can see the difference in ArcCatalog:

Faults shapefile with a defined projection

I added all of these to the map and also added the digital ortho quad image to it, after specifying its projection.
Digital orthoquad with no projection Digital ortho quad with the State Plane defined as projection

I exported the image with the projection info added to it and added this exported image to the data frame. This was the map that was produced as a result:
Map projected onto CA state plane III, NAD 27

Projecting to UTM zone 10

Then I created a new data frame to house my UTM-projected shapefiles and proceeded to project all of the CA State Plane shapefiles into UTM.

Here's a screenshot of one such projection:
Projecting census blocks from CA State Plane onto UTM

I added all of the projected files into a separate data frame, and then added the (previously exported) digital ortho quad image. This was the result:
Map of the shapefiles projected to UTM-10

You can see the difference in projections by comparing the earlier screenshot from ArcCatalog to this one:
Faults shapefile spatial metadata in ArcCatalog

Just for fun, I added the unreferenced (original) digital ortho quad image to the data frame to see what would happen. From this screenshot, we can clearly see that the original image was not very usable without a proper projection assigned to it:

UTM data frame with unreferenced digital ortho quad image

Projection to CA State Plane III, NAD 83

For the projection into NAD 83, I decided to use the batch project tool to perform the projection at once for all of the files. Here's a screenshot of the window:
Batch projection of NAD 27 files into NAD 83

This is obviously a much faster and more efficient method of projecting a group of files into a given projection. I then created a third data frame to hold the NAD 83 shapefiles, and added them to it. When adding the NAD 27 digital ortho quad image, the following dialog box popped up:
Transformation warning dialog box

I verified that the correct transformation was selected and then proceeded. Here is the resulting map:
Map projected onto CA State Plane NAD 83

Taking a look at ArcCatalog again, we can see the current projection:
Faults shapefile spatial metadata in ArcCatalog

Differences between projections

When looking at the different maps produced by projecting the same data onto CA State Plane III NAD 27, UTM zone 10, and CA State Plane III NAD 83, we can see that the same data looks completely different depending on the projection using which it is projected. This makes sense, because a projection is a way of flattening the spherical surface of the Earth and projecting it onto a 2-dimensional plane. Different projections have different formulas for accomplishing this effect, and thus the same geographic data will look different depending on the projection used for rendering it on a flat surface.