Review Projections etc. &
data types & Downloading
or
HOW TO GET CONFUSED
1
Overview
• Review of Projections
– How to do
• Difference between Define and Project
– Scale
• Data Types
– GeoDatabase
– Coverage
– Shapefile
- orthophotos
- images
• Downloading
2
Overview
• Review of Projections
– How to do
• Difference between Define and Project
– Scale
• Data Types
– GeoDatabase
– Coverage
– Shapefile
- orthophotos
- images
• Downloading
3
On the Sphere
4
On the Flat Map
Extents
-76.19,
43.07
5
X, Y = Longitude, Latitude
90
60
30
0
-30
-60
-90
-90Stretch the
0 bottom
+90
+180
Lines of constant Longitude
Lines of constant Latitude
Stretch the top
Equator
-180
6
X, Y = Longitude, Latitude
90E, 30N
Equator
90W, 30S
-180
+90, +3090
60
30
0
-30
-60
-90 -30
-90
-90
0
+90
+180
Lines of constant Longitude
Lines of constant Latitude
7
Projection data
• The .xml file is the metadata file you see
in Catalog
• ArcGIS does NOT “see” the .xml file
• It sees the .prj file
8
Xml vs. prj
• The xml file is created from the prj
• It is a one way connection
• In general, the extent of the file will
not change when you change its
projection
9
Projecting
• Two Cases and Two Tools
– Data has a .prj
• Then you can project it to another one
• Project
– Coverages are different than features
– One tool for each
– Data does not have a .prj
• Then you must DEFINE PROJECTION
• Again, two tools
10
11
Only if working with
COVERAGES!!
12
So
• Use Project if the data has a .prj
• Use Define Project if the does not
have a .prj
• Make sure you are working with the
correct tools for coverages and
features
• If wrong tool won’t display your data!
13
Last little booby trap
Geographic
SCALE
14
Not lining up?
• Obviously data that is not in the same
projection/datum is not going to line
up if there is no .prj file
• Data of different scale, even if the
same projection and datum, may not
line up very well
15
Scale Problems
• Remember that scale is related to
data accuracy
– 1/25k is + or - ~40 feet
– 1/100k is + or - ~ 80 feet
– Etc.
• Also have generalization
16
Overview
• Review of Projections
– How to do
• Difference between Define and Project
– Scale
• Data Types
– GeoDatabase
– Coverage
– Shapefile
- orthophotos
- images
• Downloading
17
Coverages
• Oldest data structure (see book)
• Are folders
• so have to move the
entire cover folder (PC)
•If the cover is in a folder
that has a INFO folder
then it is a Workstation
cover and you have to move
the entire containing folder
18
Shapefiles
• A simpler structure than coverages
• Like drawing
• Lines between polys are duplicated
(they are NOT in a coverage)
• By themselves shapes are single
precision
• In a GeoDatabase they can be double
precision
19
GeoTiffs
• Raster images (graphics)
• Geo means they know where they
belong is space
• Tif means that they are Tif images
• Used most often for images of quad
sheets and other dumb maps
20
Images & orthophotos
• Images
– Usually satellite images
– Also know where they belong
• Orthophotos
– Aerial photographs
– Rectified so are geographically correct
– High resolution now available (2 ft)
21
Grids
• DEM or digital elevation models
• Usually of continuous surfaces
• Like images and orthophotos they
know where they belong
22
GeoDatabases
• The latest whiz-bang structure from
the gnomes at ESRI
• Are containers for data
• All of the types we have discussed
• Deal with all the data at the same
time
• Allow interlayer topology
23
This week
• You will be downloading many of the
data types we have discussed
• And one more - the exchange file
–
–
–
–
E00 files
Text versions of coverages
So you have to import them (tool)
And you will probably want to convert
them to shapefiles
24
Overview
• Review of Projections
– How to do
• Difference between Define and Project
– Scale
• Data Types
– GeoDatabase
– Coverage
– Shapefile
- orthophotos
- images
• Downloading
25
Downloading
•
•
•
•
There are lots of data sites
Some are easy, some are not
Stay away from the National Map
See my GIS Links page
26
Summary
•
•
•
•
•
•
•
Coverages
Shapefiles
Grids
GeoTifs
Images
Orthophotos
GeoDatabase
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