GIS

distortion of earth's surface

*can distort true area of features *can distort true distance b/w features *can distort direction b/w features *can distort direction between features *can distort shape of features

CAD

*computer aided design (originally computer aided drafting). *used in many applications to create detailed blue prints and engineering drawings.

editing step

*digitizing *data entry *then editing *lastly: building topology, & joining tables

factors in accuracy of spatial data

*distortion due to projection choice *age *natural variation *extent * density of observations * relevance *attribute errors

choice of cell size depends on

*extent of mapped area *scale of map *degree of precision required *amount of computer power available

Geocoding concepts

*geocoding layer, address table and geocoding service. *interpolation of addresses points based on left0right address ranges *values of a data dictionary (alias table) *geocoding to centroid or random pattern with zipcode

density of observations

*scattered observations may miss important info *ex: soil maps are based on one pit for every ten acres or less

Digital Elevation Model

*this data can be used to create a variety of visualizations of the surface or as an input to other computer models *DEM data is available for the U.S from USGS

Steps needed to build a GIS

1. digitizing 2. error detection 3. editing 4. Building topology 5. adding attribute data

Large or small scale

Any map of 1:50,000,000 or larger scale 1:24,000,000 1:5,000,000 is large scale map. A map of 1:100,000, 1:250,000 or smaller scale is a small scale map.

precision

Extent to which a measurement can be consistently reproduced . The state or quality of being exact.

Attribute data

In GIS, graphics are linked to attribute data for each object. Attribute data is stored in tables.

Sampling

Technologies GPS for creating street centerlines, airborne and ground level LIDAR make dense sampling of elevation possible.

dasymetric maps

a class choropleth maps where additional information is used to map the volume of features.

contents of a field

a field contains data about a single category of things for all members of a group.

political boundaries error

a hard set of data to get right due to annexations, enclaves and boundaries being defined by property lines which are not visible on imagery

vector

a quantity with a starting point, a displacement and a direction

contents of a record

a record contains all the information about a single member of a group with respect to each category

GIS

a suite of computer programs designed to store data about geographic features linked to descriptive attribute data

geoid

accounts for the high and low spots and flattened deformed spheroid that is our earth *shape of earth

raster

cell-based, digital imagery * raster is superior for continuous data

Coastlines

classic case where simplification is needed for generalization when scale of maps change

3 main types of projections

conformal: shows directions between features correctly are maintains shapes over small areas equal area: areas of objects are equal from top to bottom equidistant: distances between features are preserved

more on raster

consists of a regular tessellation of space usually raster employs square grid cells like a tiled floor

data capture

converting data into a form where it can be used by a GIS usually from paper maps into digital topological GIS data.

relevance

data should be as relevant as possible. ex: data should be as relevant as possible

Datum

define the size and shape of the earth and the origin and orientation of the coordinate systems used in maps and geospatial data

manual GIS

developed by Ian McHarg starting in 1950's using multiple Mylar overlay maps for urban planning

label or DOT distribution maps

dots are used to show location and sometimes number of some feature. labels and/or annotation can be added and scale reanges and reference scales. dots can be graduated or proportional

contours

elevation data can be represented with contours

digitizer tablet

employed to trace line work. once an object has been entered, associated attribute data is keypunched in

shapefile

file format created for ArcView in 1990's. Less sophisticated geodatabase format of arcGIS.

National Map Accuracy Standards

for large scale (1:24,000 topo maps) the horizontal accuracy standard is no more than 1/30th of an inch off the map = 75 ft on the ground.

ESRI (Environmental Systems Research Institute

founded in 1969 by Harvard Alumni Jack Dangermond to work on image processing for NASA Arc/info developed

street addressing/ geocoding

geocoding layer and service is needed to render a street address into a geographic location via the process of geocoding

object oriented data-base structures

geodatabase structure which is obejct oriented thus there can be one to many and many to many relationships between geographic features and attributes

arc-gis online

has the advantage that you need not download, decompress, and import geospatial data. you can see your existing layers in relation to new data re-projected on the fly

GIS

has two methods raster & vector

hydrographic data errors

has very poor feature name accuracy in the USA

Issues with sampling

if the diversity of an area is large then more intense sampling is needed

Small scale

implies lack of precision but it could be more accurate

Large scale

implies precise information but not accurate

linear geo-referencing

includes street addresses but also includes pipeline data and highway mile markers

data structures

is how data is stored in a GIS

joining related tables

joining a stand alone table to an attribute table requires there to be a primary key in common.

overlay

layer over a layer, they often include flood plains, protection for water wells and protection for historic districts

infrastructure errors

limited infrastructure data in DLG including mines, roads, schools, & many houses, but only if visible on aerial photography

query of a table

logical expression that selects a set of records that match the criteria

attribute data

lots of errors are common, key punching errors

heads up digitizing

maps are scanned and the image displayed on the computer and then a GIS technician follows the outlines on the screen using a cursor to trace objects and keypunches the attribute data

precision of spatial data

maps derived from aerial imagery are only about as accurate as twice the resolution

projection

mathematical method by which features on the spheroid of the earth are represented in two dimensions

spheroids or ellipsoids

mathematical representations of earth need to be used in GIS

discrete objects

might include a well, a business location, or a crime incident or a school

data types

most common types of data are numerical, character, date

arcgis online

most of the data is posted by users with no QA/QC by ESRI. *metadata can be lacking and the data can be in error mislabeled and incomplete

More on Attribute tables

must contain unique feature identifier (FID) that links to feature on map *also has topological information

important datums

north american datum of 1927 north american datum of 1983

scanner

photo optical device that measures reflected laser light and stores the output as a raster format image file

examples of vector data

polylines, contours, roads, streams, polygons, parcels, vegetation, points *discreta data

Map Scale

relationship (or ratio) between distance on a map and the corresponding distance on the ground. For example on a 1:100000 scale map, 1 cm on the map equals 1km on the ground

layers in raster

same coordinate systems, grid size, and orientation so that they can be set down on top of each other

problems with scanning

scanned maps are only raster images. The computer has no idea what the light and dark dots signify

isopleth

shows lines of equal value for some property

soil errors

soil series maps are not available in digital form in most cases and data is fuzzy

Natural variation

soils, geological mapping units, wetlands, boundaries of bodies of water, and vegetation are all fuzzy or change over the year.

continuous data

soils, geology, land use, land cover, land ownership

cadastral data

some states have state wide data but not all

precision vs accuracy

spatial data can be precise but not accurate, however if spatial data is not precise

TIN (triangulated irregular networks)

special case used for simulation of terrain

raster structure

stored in rows and columns data with differing values for a theme are assigned different values in each grid cell. raster data is continuous

cartogram

the area of feature polygons is adjusted by the magnitude of the theme being mapped. often used for numerical data not categorical data

area class maps

the boundaries are determined not by some independent variable like the existence of a nation, county, census tract or but by the data itself. ex: soils maps

Database Tables

the database is made up of rows and columns like a spreadsheet. The rows are called records, the columns are called fields.

distortion

the most important concept related to projection is distortion

topology

the relationship between different spatial objects

cardinality

the relationship between items in two tables *records could be related one to one, one to many, many to one, or many to many. *it is determined by the destination table

the primary key

the typical contents of the first field it is important to link tables to graphics and one table to another.

chloropleth maps

thematic map where value of properties within a boundary are classified and that set of shades is applied within the boundary

complex raster structures

there are hexagonal and triangular grids

SQL

to query data in a relational database a structured question must be asked. *a programming and command language called structured query language has been developed to ask questions of relational databases

TIGER

topologically integrated geographic enconding & referencing system

buffer zones

tracking what is in the adjacent cells is also important a type of spatial analysis

accuracy

truthfulness of information

chloropleth maps

use existing boundaries and use fill colors or patterns to differentiate these and or show values for these areas

stand-alone tables

useful if data is being periodically updated *requires a unique primary key an object ID (OID) *have no tie to map features

boolean operations

value stored in a table that instructs arcgis to join data together according to rules developed by english mathematician George Boole

polygon overlay

values in each overlapping cell in each layer can be combined mathematically to create new layers.

generalization

when data of differing scales or projections is combined it must be transformed. the change from a large scale to a small scale- generalization

extent

you may not have data covering the entire area of interest