1

Review Projections etc. & data types & Downloading

or

HOW TO GET CONFUSED

2

Overview• Review of Projections

– How to do• Difference between Define and Project

– Scale• Data Types

– GeoDatabase - orthophotos– Coverage - images– Shapefile

• Downloading

3

Overview• Review of Projections

– How to do•Difference between Define and

Project

– Scale• Data Types

– GeoDatabase - orthophotos– Coverage - images– Shapefile

• Downloading

4

Syracuse

On the Sphere

5

On the Flat Map

-76.19, 43.07

Extents

6

X, Y = Longitude, Latitude

Lines of constant LongitudeLines of constant Latitude

0-90 +90-180 +180

0

-30

30

-90

90

-60

60

Equator

Stretch the top

Stretch the bottom

7

X, Y = Longitude, Latitude

Lines of constant LongitudeLines of constant Latitude

0-90 +90-180 +180

0

-30

30

-90

90

-60

60

Equator

90E, 30N

90W, 30S

+90, +30

-90 -30

W76.15° N43.04°

-76.12° 43.08°

8

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

9

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

10

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

11

12

Only if working withCOVERAGES!!

13

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!

14

Last little booby trap

GeographiGeographic SCALEc SCALE

15

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

16

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

17

Overview• Review of Projections

– How to do• Difference between Define and Project

– Scale• Data Types

– GeoDatabase - orthophotos– Coverage - images– Shapefile

• Downloading

18

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

19

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

20

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

21

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)

22

Grids

• DEM or digital elevation models• Usually of continuous surfaces• Like images and orthophotos they

know where they belong

23

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

24

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

25

Overview• Review of Projections

– How to do• Difference between Define and Project

– Scale• Data Types

– GeoDatabase - orthophotos– Coverage - images– Shapefile

• Downloading

26

Downloading

• There are lots of data sites • Some are easy, some are not• Stay away from the National Map• See my GIS Links page

27

Summary• Coverages• Shapefiles• Grids• GeoTifs• Images• Orthophotos• GeoDatabase