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