## 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 (early 2020's), the US will go to 4 new datums (one for each tectonic plate), 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 selected point.
• Get the values at uniform spacing over the map.
• Export the values into a database, which you can then visualize.
• You can also look at the MGRS zones and 1:100K grid squares from that menu.  These only work for small scale maps.

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 parts.

• Determine the pattern of distortion on each from the Tissot indicatrices.  You will need a map and a discussion for this.  overlay Tissot indicatrices from the Cartography menu choice of a map menu
• 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 Mercator projection.
• Look at horizontal datum shifts (Cartography. Datum shift), 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, which you can get with Info).  Can you adjust the map size or coverage area to see the changes?

### 3.  Projections and UTM and MGRS Coordinates

Answer the following questions, with screen captures as appropriate.  Consider just your home county TIGER data (2017 edges):

• Cylinndrical maps
• Mercator projection.  Turn off the option to switch to the UTM projection when you zoom in (Options, Vector maps, Auto shift Mercator to UTM).
• UTM projection, in the correct zone, which you should pick before the map opens. (Or you can start with Mercator, and turn the Auto shift Mercator to UTM back on).
• 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 not.
• What is the distortion pattern on the Mercator and UTM maps?  overlay Tissot indicatrices from the Cartography menu choice of a map menu.
• How do the two conic projections look different from the two cylindrical maps, and how much difference can you see between the Albers and Lambert conics?
• 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?

Detailed directions:

• New Vector Map.  You can do this for multiple projections.  You might want to open all four maps in windows at the same time.
•   Database overlay to pick the TIGER shapefile of your home county. If you set the option to plot the database on all maps (Options, Database, 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, Map Overlays, World outlines can turn them off.
• Grid to add UTM grid and lat/long graticule.  The UTM grid will not appear until you zoom in.
• 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 with a UTM projection 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
• Cartography. Datum shift
• 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 options form.