Review Projections etc. & data types & Downloading

FOR357/557 Review Projections etc. & data types & Downloading or HOW TO GET CONFUSED

Overview • Review of Projections • How to do • Difference between Define and Project • Scale • Data Types • GeoDatabase - orthophotos • Coverage - images • Shapefile • Downloading

Syracuse • On the Sphere

On the Flat Map Extents -76.19, 43.07

Stretch the top Stretch the bottom X, Y = Longitude, Latitude 90 60 30 Equator 0 -30 -60 -90 -180 -90 0 +90 +180 Lines of constant Longitude Lines of constant Latitude

-76.12° 43.08° W76.15° N43.04° X, Y = Longitude, Latitude 90E, 30N +90, +30 90 60 30 Equator 0 -30 -60 -90 -30 90W, 30S -90 -180 -90 0 +90 +180 Lines of constant Longitude Lines of constant Latitude

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

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

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

Only if working with COVERAGES!!

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!

Last little booby trap Geographic SCALE

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

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

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

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

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

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)

Grids • DEM or digital elevation models • Usually of continuous surfaces • Like images and orthophotos they know where they belong

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

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

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

Coverages Shapefiles Grids GeoTifs Images Orthophotos GeoDatabase Summary

Review Projections etc. & data types & Downloading