when you begin a project, a reference data layer is placed on the map first. this initial layer(s)...
TRANSCRIPT
When you begin a project, a reference data layer is placed on the map first. This initial layer(s) is called the base map.
There are different types of base maps exhibiting variousinformation and accuracy.
In this lesson, you will learn some of the major characteristics of These base maps.
You will also learn about the different types of Map ProjectionsAnd how map projection affects the display of data layers.
Lab 2: Base Map Types & Map Projections
Geographic Base Map:
The computerized data layer(s) that contain all geographic (spatial) objects that are common to user(s).
The geographic base map is the first layer(s) placed on the GIS display. Other data layers are then placed above and used for analysis in project work.
THE 3 MOST COMMON BASE MAP FILES
1. TIGER Line
2. CADASTRAL
3. PLANIMETRIC
First major Base map Type: TIGER/ Line files
- Electronic map files (layers) that display various map features
- Developed by U.S. Census Bureau to aid in the census (counting population)
- Topological Integrated Geographically Encoding & Referencing
- Covers the entire United States
USGS Topographical MapTIGER spatial objects captured from USGS 7.5 min. quadrangles
Quads were about 50 sq. mi. (7x7miles)Scale 1:24,000 (1 inch = 2,000 feet)
Features are displayed on the USGS quads (roads, rail, water, contours, etc.)
7.5 min USGS Quad Index for NE Ohio
57,000 Quad Maps in US
Features digitized &spatial objects created in TIGER
7.5 min quad
Example Data Layers (spatial objects) in TIGER 2000
Example Data Layers (spatial objects) in TIGER 2000
Demographic data captured in the Census can be linked to each block & displayed or analyzed.This attribute data is available through the US Census Bureau. dBase files or excel spreadsheets can be created from the attribute data.
Second Major Base map Type: Cadastral- official record describing/ recognizing the legally defined registration of land (property)
- often referred to as the “parcel base map”
- created by County Auditor or Tax Assessor
- used for tax assessment, public services, zoning, etc.
Based on Legal definitions of parcels (property)ie. From a surveyed x,y location, move North 62 degrees East for 157 feet, move South 91 degrees West, move … until reaching the original x,y location thus creating a parcel polygon (area)
Cadastral Base Map Example
Paper Maps are contained in
292 Books in Cuyahoga Co.
About 30 map pages per book 8814 individual paper map pages
Paper maps digitized
Information is historical
Digitized as lines NOT polygons
Cadastral Map: layer examples
Parcel polygons created paid for: by Cleveland Water Dept.
Currency: Complete through Nov. 1994
No new parcel splits/combinationson digital parcel data layer since 1994
County has been in process of updatingparcels to the present- not complete for various reasons
County to release updated parcel filein 2007
Auditor also maintains attribute databases containing informationabout each parcel
NODIS has an “enhanced” parcelpolygon file for the county (1994)
Third major Base map Type: Planimetric- Physical features that can be seen on ground are spatially
recorded (mapped)
- Examples of features captured are road edges (curbs), building footprints, railroads, 2 & 10 foot elevation contours, spot (point) elevations, retention walls, driveways, bridges, interstates, recreation areas, and trees.
- Cuyahoga Co. Engineer’s Office – funded the process to create the planimetric layers
- Planimetrics are created from aerial photography
Aerial Photography example
Aerial photography distortions caused by:
- Uneven surface of Earth distorts feature elevations and shapes
- Tilt of airplane can cause photo distortion of features
Orthogonal Rectification:- process to correct the aerial photo distortions
- uses mathematical algorithms to correct
- a very technical, detailed, & complex process
After aerials are rectified:- orthophotos are generated with corrected shape, elevation, and spatial location of the features captured
- There are 1110 orthophotos for Cuyahoga Co.
- orthophotos are spatially located, placed side by side similar to putting a puzzle together
Planimetric data layers are then electronically captured and placed into the correct data layer. Ie. Road edge, building footprint, etc.
Examples of Planimetric data layers
What Base Map do I use???
- depends on user application or project
The base map used can be:- 1 or more data layers within one base map type
- or a combination of data layers from more than one base map type
Combination Base Map Example: Planimetric- road edge & Cadastral- parcel
Map Projections:
- The process of converting 3 dimensional features to 2 dimensional objects.
- Over 50 major map projections with many subtypes
- 2 major projection types deal with:
Equal Area
Direction
Map Projection Examples
Equal Area Projectionsattempt to display featureswith lowest amount of area distortion
Direction ProjectionsAttempt to display featuresFor navigational purposes(To get from point A to B)
Direction Projection
Latitude- Longitude
- is NOT a projection
- actual measured angles from the center of the earth
- grids are equal size
- stretches US, Anactica, etc.
U.S. State Plane- Northern Ohio NAD83
- best use is local maps in Northern Ohio
- units are in feet
Any projection type is acceptable to use whenproject work is in a small geographic area.
Large scale maps = small geographic area ie. 1 inch = 50 feet
Small scale maps = large geographic areaie. 1 inch = 50 miles