gis midterm 1

GIS

computer-based framework for collecting, analyzing & visualizing spatial information

spatial information

data that includes a geographic or location component

how gis is used in ecology

plan where and how to collect data, to make measurements and calculations, to create new variables, link data together for spatial analysis,

what is a map

a symbolic representation of an area that depicts geographic features and their relationships, often using symbols and scales

what is the use of maps?

served to store geographic knowledge, visualize spatial patterns, guide data collection,
analyze ecological processes, and communicate findings

geographic features

information that represents the real world

cartographic elements

information that helps the reader to interpret the geographic features

reference map

generalized map that shows general spatial properties like world maps and road maps

thematic map

highlights a specific theme by emphasizing spatial variation (feature, choropleth, dot density and contour maps)

chloropleth map

shows quantitative information for areas (maps of population density)

dot density map

shows quantitive information with dots (dot = value)

contour map

shows line of equal value (elevation, rainfall, temp)

spatial resolution

smallest feature that is individually resolved (high means smaller things are captured)

temporal resolution

frequency of measurements or the shortest time interval a system can capture to distinguish between events

attribute resolution

How fine are distinctions between categories

scale

shows relationship between map units and real units

ratio

1:24000 means one display unit = 24000 real units; 3 cm on a 1:100,000 scale is 300,000 cm

advantage of a scale bar

it remains accurate and functional even when a map or drawing is resized

scale trade-off

as extent increases, resolution decreases

large scale

more zoomed in, more information

storymap

digital storytelling with maps

mercator projection pros

preserves shape and direction, a straight line gives the correct angle to follow between 2 points, close up views match real-world angles

mercator cons

distorts area (poles enlarged), reinforces eurocentric perspectives

galls-peters projection pros

preserves size (equal area)

galls-peters projection cons

distorts shape (countries stretched vertically)

conformal pros and cons

preserve angle and shape, distorts area and distance

equal-area pros and cons

preserve area, distorts shape

equidistant

preserve distance, distorts direction, shape, and area

azimuthal

preserves direction from a central point, distorts distance, shapes, and area

coordinate system

defines the shape and size of the earth with an ellipsoid

reference ellipsoid

mathematical model of the Earth’s shape; WSG-84

datum

reference system that aligns the ellipsoid to the Earth’s surface using a network of reference stations with fixed coordinates; defines origin and orientation of coord system; NAD83;

geographic coords

uses lat and long on 3D model of earth

projected coords

uses linear units (Easting and Northing) on a 2D map

Coordinate Reference System (CRS)

full spatial reference framework for the data (coord system, datum, projection)

spillhaus projection

azimuthal projection stitched together to display the ocean as a continuous body of water; useful for studying migration routes of marine species, marine pollution, fish stocks, etc.

vector data layer

stores data as individual features; attribute table with data about each feature; points, line, and polygons;

raster data layer

stores data as an array of cells in rows and columns, each cell has a value (one only), layer properties contain information on cell size, number of cells per row/col, location of one cell

vector rules

all rows in a layer must be the same type, features within a data layer can overlap

raster rules

cells within data has a consistent size, data layer has info over one variable only, cells cannot overlap in space/time

what should we use raster layers for?

background imagery, continuous surfaces

what should we use vector layers for?

features we want to label and measure

lakes with vector vs raster

vector good for when you care about individual lakes, their exact boundaries, and their attributes. raster good for to see how the lake gets bigger or shrinks over time

buffer tool

Creates buffer polygons around input features to a specified distance

dissolve tool

Merges polygons that share a common attribute, removing their internal boundaries

intersection tool

Keeps only the areas where two layers overlap

union tool

Combines two layers into one, keeping all areas and their attributes

difference tool

Subtracts one layer from another.

Why is it important to keep GIS data organized in a single folder?

Because QGIS project files only store references (addresses) to data, not the data itself. If data is moved or renamed, the project link breaks

Example naming convention: World_Cities_WGS84_LL.shp

Name = World_Cities; Datum = WGS84; Coordinate format = Latitude/Longitude

What are the three essential files in a shapefile?

.shp (geometry), .shx (index), .dbf (attributes)

What does the .prj file store?

projection info

what is a shapefile

A common vector data format made of multiple files (.shp, .shx, .dbf, etc.) that store geometry and attributes

What is the difference between cartesian and ellipsoidal distance in QGIS?

Cartesian = flat ruler on map (projection-dependent); Ellipsoidal = string over globe (accounts for Earth’s curvature)

Which distance measurement is more accurate for global data, and why?

Ellipsoidal, because it accounts for Earth’s curvature

Why might you still want an equidistant projection even if ellipsoidal distances are available?

Because map readers interpret distances visually on the flat map, equidistant projections reduce misinterpretation .

What’s the difference between a reprojection and a datum transformation?

Reprojection = changing coordinate system (projection); Datum transformation = aligning datasets with different datums (same projection)

What are five main ways to create new data layers in GIS?

1) Create new features manually in QGIS, (2) Subset features from existing layers, (3) Import coordinates from spreadsheets, (4) Digitize from imagery, (5) Import GPS waypoints/tracks

Why is clipping layers to a study area useful?

It reduces file size, speeds up processing, and focuses analysis on the area of interest

Why add Easting and Northing fields when creating a new point layer?

To store coordinate values directly in the attribute table for reference and quality control

What is digitizing in GIS?

Manually tracing features (lines, polygons) from imagery into vector layers

Why convert a .kml GPS file to a shapefile with reprojection in QGIS?

To make it permanent, match the project CRS, and apply necessary datum transformations

What’s a key difference when importing .gpx vs .kml files in QGIS?

gpx may contain multiple layers (tracks, routes, waypoints) while .kml often stores a single layer