GEOL 452/552 - GIS for Geoscientists I Lecture 15
Lecture Plan Midterm Multiple choice part graded (+ solutions) Jump to Ch. 11 for one lecture Coordinate systems Projections, datums, spheroids Unprojected (geographic) coord. syst., UTM On the fly projection vs. data file projection (Re)project data vs. set projection (ArcTools) Tutorial Ch11: 1-29 (35-46 shows how to georeference an image file)
How to draw features from a round sphere... Projections - refresher... on a flat map? Latitude/Longitude 20.44395, 45.23432 20.44732, 45.28453...
Coordinate System/Projection All points on a sphere are measured in angles Latitude (N-S), Longitude (E-W) Draw a set of points (Iowa - Des Moines) on paper? A) Unprojected ( GCS ) - no conversion of Lat/Long numbers of each point* Distortion of shapes increases when going North B) use a Projection: Follow along: Ch11_class_ex \LatLong_vs_UTM.mxd (layout view) Lat = 43.567 DD Long = -93.698 DD Translate Latitude/longitude angles (degrees) to distances (meters) Aim: less distortion (locally) for angles, distances Distance computation (ArcGIS) work better with projected coordinates (meters instead of degr.) UTM x = 450460 m UTM y = 4621850 m
Unprojected (geographic coordinate system, WGS 84 datum): Angle based grid: perfect squares, right angles
Projected coordinate system (UTM) Angle grid: distorted round rectangles Internally meter based
Projected coordinate system (UTM 15N) meter based grid: perfect square*, right angles Degree (angle based) grid
Projection: mathematical transformation ( formula ) to go from spherical (geographic) coordinate system (ArcGIS: GCS) to planar coordinate system ( round to flat ) Three ingredients for transformation Type of Spheroid Type of Datum Type of Projection system
Type of Spheroid used Spheroid = sphere (3D) with different axis length Geoid, more complex, true shape of the earth But: We can locally approximate a geoid with a spheroid Common spheroids: Clark 1866 Spheroid GRS80 Spheroid Here: Good fit of simple spheroid model with true 3D globe (geoid) for North America + Spheroid center
Sphere Spheroid Datum = Center of Spheroid (offset) Geoid Datum
Type of Datum Datum: 3D center of the simple approximation (sphereoid) of the Earth Difference (correction) from the true center of the Earth (Geoid) In North America: North American Datum (NAD) NAD27 implies Clark1866 spheroid NAD83 implies GRS 1980 spheroid Geographic coord. systems (GCS) also need a Datum (e.g. WGS 84)
UTM system Very common type of projected coordinate system Let s explain the acronym, but backwards (M-T-U) Mercator means cylindrical projection Named after Gerhard Kremer, a Flemish cartographer who lived from 1512 to 1594. Gerhardus Mercator was the latinized form of his name. used this projection for a map in 1569
Cylindrical (Mercator) projection
T is for transverse Means cylinder is rotated to the side make one N-S slice per UTM zone 1 UTM zone = 6 degree wide slice (yellow)
U is for Universal meaning: works (nearly) everywhere (except poles) location as x/y meter based offset from artificial origin 6 or 7 digits for coordinates
On-the fly projections: ArcMap conversion of a layer into the dataframe s coordinate system Example: New Dataframe, add a unproj. (GCS) layer Change Dataframe to UTM 15N - NAD83 Layer will be on-the-fly projected and drawn as UTM15 (file is internally still unprojected) But: Can be imprecise, better: permanently project into a common System using the Project Tool
Tool: Project (Re)project an already projected layer Example: Convert a file from Stateplane system (conic projection, NAD27, feetbased) into UTM (cylindric projection, NAD83, meter-based) Will recalculate all the features coordinates Coordinates inside data file are permanently overwritten (!) Use Projections and Transformations - Project Tool in ArcToolbox (Think reproject)
Tool: Define Projection Only use for data that has no coordinate system (i.e. undefined )! Example: you have some GPS coordinates (Excel), you know that your GPS recorded them as UTM 15N - NAD 27 Projection header undefined GPS points: 445678,654321 445021,650001 444823,649200 A) give coordinate system during import of x/y points into Arc (we ll do that in chapter 11/12) B) Use the Define Projection tool in ArcToolbox (set projection) Will not re-write the actual data, only the file s projection header
? Datafile Projection info ( Properties ) GPS points: 445678,654321 445021,650001 444823,649200 Define Projection GPS points: 445678,654321 445021,650001 444823,649200 no change inside data file! ROADS -103.567,44.628-103.678,44.653-103.765,44.732 Projection STATE 445678,654321 445021,650001 444823,649200 data inside file has changed! unprojected (angles) projected (meters)
Wrap up Ch 11 tutorial steps 1-29 (steps 35-46 shows how to georeference an image file) HW 9, Ch 11: Ex.3 (3 pts) In addition to (after) Ex. 3 (6 pts) perform these steps: Show lower 48 states (in original coordinate system) make a screenshot How wide (East-West, in meters) is UTM 14 in North Dakota, how wide is it Texas? Set Data frame to Unprojected (GCS) - NAD 83 Make another screenshot of lower 48 (in GCS - NAD83) Again compare the width of UTM 14 (N. Dakota vs. Texas) but this time in degrees Which UTM zones cover Iowa and where do they start and end (East-West direction only, in degrees)? (3 pts)