MICRODEM GIS Course Lab 2: Projections and Horizontal Datums
This lab will look at three things:
- Map projections, and how they handle distortion.
- Horizontal datums, and the shift going from one datum to another.
While at present almost all data will be in NAD83 or WGS84, this was not the
case when I started teaching and a lot of data was in NAD27. In the
near future, the US will go to a new datum, and while the shift will
probably be less, you will have to worry about it for precise work.
- UTM and MGRS coordinates
In looking at map distortion, we will use the
Tissot indicatrix. This is a
circle on the ground, and we can look at it on the map and see what distortion
occurs. We can also look at the values for h and k, the distortion in the
two directions. For many maps, we may need to look at the 4th decimal
place to see any changes.
The tools for both the datum transformation and map distortion are on the
Cartography menu, and may
give you several options:
- Click on a location, and see the desired characteristics at the
- Get the values at uniform spacing over the map.
- Export the values into a database, which you can then visualize.
Download TIGER data (2016 edges) for your home county.
You will have to pay close attention to several things for this lab to work:
- The projection of the map
- The datum you request, primarily for the second datum.
You should clearly label any output files your create, and any maps.
The computer term
GIGO applies: looking at datums shifts from WGS84 to NAD27 in western Europe will not
reveal anything very interesting.
1. Planar projections
Look at the following projections: stereographic, orthographic, and Lambert
azimuthal equal area. Except for the polar stereographic, these
projections are only appropriate for looking at an entire hemisphere. Make a table with your answers to the first two
- Determine the pattern of distortion on each from the Tissot indicatrices. You will need a map
and a discussion for this.
- What is the shape of meridians and parallels on each, and how do they
intersect on the map?
- Look at an earth rotation movie for each projection, paying particular
attention to Australia's size and trajectory.
- Which map is best, and do you think we need all three?
2. Datum shifts
New Vector Map, and insure you have a
- Look at horizontal datum shifts (Cartography.
at the following subset
areas. For each subset area, consider the changes for both UTM and
geographic coordinates, and any changes over the area requested area (corners and
center would be a good starting point). Put these results in a table.
You will probably need to edit the table created by MICRODEM (save as CSV or
HTML or DBF, all of which Excel will open).
- CONUS, compare NAD27 to WGS84
- North and South Korea, compare Tokyo datum to WGS84
- France, Germany, Italy, Spain, and Portugal, compare European
Datum 1950 to WGS84.
Grid Have the GIS program put graticules for both datums on the maps;
can you see the difference? Why or why not? Consider the magnitude of
the datum shift, and the scale of the map (or size of a pixel).
3. Projections and UTM and MGRS Coordinates
Answer the following questions, with screen captures as appropriate.
Consider just your home county TIGER
data (2016 edges):
- UTM projection, in the correct zone, which you should pick before the
- Mercator projection. Turn off the option to switch to the UTM
projection when you zoom in (Options,
Vector maps, Auto shift Mercator to UTM).
- Two conic projections (conformal and equal area). You might not be
able to get the UTM grid to appear until you zoom in.
- Can you readily see the difference between the UTM and Mercator
projections? Why or why
- What is the distortion pattern on the Mercator and UTM maps?
- How do the two conic projections look
different from the two cylindrical maps, and how much difference can you
see between the two?
- Make a table for the four projections for your county, showing:
- Are the grids and graticules "square" or "rectangular"?
Why do the shapes vary?
- Are the grids and graticules parallel with the edges of
the maps, or rotated? Why?
- Get the geographic, UTM, and MGRS coordinates for your home
(Calculate, Point coordinates).
Do these vary depending on the projection you use?
New Vector Map. You can do this for
multiple projections. You might want to open all four maps in windows.
Database overlay to pick the
TIGER shapefile of your home
county. If you set the option to plot the database on all maps (Options,
Plot on all maps), it should appear on all four.
- If you zoom in to the point where the world outline vectors overlays
become annoying in their inaccuraccy,
World outlines can turn them
Grid to add UTM grid and lat/long graticule.
The UTM grid will not appear until you zoom in.
MGRS zones or
1:100K squares (UTM) can show the
UTM/MGRS on smaller scale maps.
Information on the map, shows pixel
size. If the map covers a large area, this could a rough average
because the size could vary throughout the map.
Open shapefile map with the
TIGER shapefile of your home county
would give you a fast map already zoomed to a good scale, but in this case
we want to compare different projections. For future labs this is
a good option.
Getting coordinates of a point. Understand that the accuracy of the
resulting coordinates depends on the dexterity of your mouse motions, and the
pixel size. On a map of the world with 10 km pixels, your coordinates are
at best only accurate to within 10 km, and maybe more if you cannot click on the
exact pixel you want.
- Calculate, Point coordinates
- Watch the mouse move and the coordinates will be on the
status bar. The coordinates of your current cursor position, with the horizontal
datum, and the elevation. The
default units are in latitude and longitude; you can also pick UTM or MGRS
on the Units tab of the
options form. Additional panels will show supplemental
information, like a secondary datum or a second choice of coordinates,
which you can pick on theCoordinates
tab of the
Finding an address
Finding the UTM Zone.
MGRS zones. The MGRS zones consist
of the UTM zone (1-2 numbers, from 1 to 60) and a letter for the latitude
- If you have too local a map, all in a single zone, this might not
- If you have a global map, you will probably have to zoom in so there
is enough room for the zone labels.
- Change the Location setting on the
Units tab of the
options form to UTM or MGRS (but not the short
MGRS), and then move the mouse over the location.
- Pick Calculate,
Point coordinates from the menu,
and then double click on the desired point.
Zooming the map to your county
Zooming to your streeet
Last revision 1/20/2017