Gis Midterm Questions

Front 1

How are GIS software different from most other software?

Back 1

GIS software allow us to look at spatial data, something other softwares do not. It helps analyze relationships and trends among said spatial data. In some cases it is the only way spatial-related problems can be solved. GIS is particularly helpful in addressing environmental issues by locating the source, location and extent of adverse environmental impacts.

Front 2

How is an entity different from a cartographic object?

Back 2

An entity is a real-world item or phenomenon that is represented in a GIS system or database whereas a cartographic object the digital representation in GIS of said real-world entity.

Front 3

Describe the successive levels of abstraction when representing real-world spatial phenomena on a computer. Why are there multiple levels, instead of just one level in a spatial data representative?

Back 3

Moving down the successive levels of abstraction, you first pass through the data model, which is a set of polygons or closed areas that record the edges of distinct land uses. The next level is the data structure, which has coordinates for all of the polygons and then the machine code stores everything in binary. While it gets less recognizable as it travels through levels, it is computer-compatible.

Front 4

Define a data model and describe three primary differences between the two most commonly used data models.

Back 4

A data model is a method of representing spatial and aspatial components of real-world entities on a computer. The two primary models are vector and raster models. A vector model is most like a typical map that we think of. It has points, line and polygons. Size and dimensions of the entity are not important in these models, only the location. A raster model divides the area into a grid-like pattern and every square has a value. This makes it much bigger than a vector model in terms of file size. It is the natural way to represent continuous data. Raster models are relatively simple, compared to vector models and are east to modify or program. Vector models can have continuous curves but are poor for images, whereas raster models are good for digital images but have stair-step curves.

Front 5

What is topology and why is it important? What is planar topology and when might non-planar be more useful than planar topology?

Back 5

Topology is shape-invariant spatial properties of line or area features such as adjacency, contiguity, and connectedness, often recorded in a set of tables. It is important because it greatly improves the speed, accuracy and utility of many spatial data operations. Planar topology assumes that all features occur on a two-dimensional surface. There cannot be any overlaps. Non-planar topology allows for overlaps and assumes it is mapping a three-dimensional world. This might be useful for mapping bridges.

Front 6

What are the respective advantages and disadvantages of vector data models versus raster data models?

Back 6

Raster models has continuous data, are relatively simple to modify, need much more storage space, may be slow, their floor is set by cell size, good for images but may have stairstep edges.
Vector models are ususally complex and hard to modify. They can be compressed much more, they are preferred for network analysis, are only limited by positional measurements and are map-like with continuous curves.

Front 7

Under what conditions are mixed cells a problem in raster data models? In what ways may the problem of mixed cells be addressed?

Back 7

Normally each cell receives one value, so when there are multiple values for one cell, difficulties arise. One solution is to simply represent only one feature. This means there may be data loss. Another solution is to make the cells small enough so that there will ever be multiple features in one cell. This means the data set may have to get a lot larger. There are ways to record multiple instances in one cell, but it may slow access.

Front 8

What is an ellipsoid? How does it differ from a sphere? What is the equation for the flattening factor?

Back 8

An ellipsoid is a three-dimensional figure whose plane sections are ellipses or circles. The equatorial radius is always greater than the polar radius. A sphere is a perfect circle in which both radii are equal. The flattening factor is: (a-b)/a

Front 9

Why do different ellipsoids have different radii? Provide three reasons.

Back 9

Originally, geodetic surveys were separated by large bodies of water so this affected measurements. Ellipsoids were also fit for each country, region or area being surveyed and there isn’t one best ellipsoid for every region on earth. This meant there were different radii. New technology has created new best ellipsoid that averages differences all over the earth, but different ellipsoids are better for different areas of the earth.

Front 10

Can you define a parallel or horizontal meridian in a geographic coordinate system? Where do the horizontal and vertical zero lines occur?

Back 10

A meridian is simply a line of constant latitude or longitude, typically longitude. That being said, it runs east to west or north to south and in a Mercator or Transverse Mercator, it is the line of intersection and the line of least distortion. In a vertical meridian, it typically starts and stops in Greenwich. The horizontal meridian hits the Earth at the equator.

Front 11

How does magnetic north differ from the geographic North Pole?

Back 11

Magnetic north is where the compass points, whereas geographic north is the northern point of the Earth’s axis of rotation. If you were standing on the geographic North Pole, your compass would point approximately in the direction of northern Canada, towards magnetic north, some 600 km away. A compass does not point to geographic North when observed from most places on earth. The compass will usually point east or west of geographic north, defining an angular difference in direction to the poles. This angular difference is called declination and varies across the globe.

Front 12

Can you define a datum? Can you describe how datums are developed?

Back 12

A datum is a reference surface. A geographic datum consists of two major components. The first components is an ellipsoid with a spherical or three-dimensional Cartesian coordinate system and an origin. Eight parameters are needed to specify the ellipsoid: a and b to define and size and shape of the ellipsoid, the x,y and z values of the origin and an orientation angle for each of the three axes. The second part of a useful datum consists of a set of points and lines that have been painstakingly surveyed using the best methods and equipment and an estimate of the coordinate location of each point in the datum. Datums may be made by creating benchmarks that have precise coordinates and they are mapped. As better technology is created, more precise measurements can be made.

Front 13

Why are there multiple datums, even for the same place on Earth? Can you define what we mean when we say there is a datum shift?

Back 13

Actual points on the Earth’s surface vary from the ellipsoid, so an ellipsoid that works for a few particular points may not work for others. This may require different datums and this is what causes datum shifts, in which a new origin for the ellipsoid is set and the coordinates measured for the first ellipsoid must be set relative to the new datum. There have been large shifts in the past, especially between NAD27 and NAD83 as new technology has been invented and better measurements have been made. There have been subsequent datum shifts but as a general rule, the changes become less and less as measurements become more accurate.

Front 14

Why do we not measure vertical heights relative to sea level anymore?

Back 14

Sea-level heights vary over time. Mean sea-level, even averaged over several decades, varies across the globe due to several factors, for example, persistent differences in water density with temperature and salinity, or regular ocean currents which ay persistently raise or lower the surface in ocean regions. This means that the mean sea level is not constant relative to the geiod or ellipsoid and will be different at Miami than New York. Instead of using sea level, they now use precisely measured benchmarks.

Front 15

What is a developable surface? What are the most common shapes for a developable surface?

Back 15

Most map projections are based on a developable surface, a geometric shape onto which the Earth surface locations are projected. Cone, cylinders and planes are the most common surfaces.

Front 16

Can you describe the State Plane coordinate system? What types of projections are used in a State Plane coordinate system?

Back 16

The State Plane coordinate system is a standard set of projections for the United States. The State Plane coordinate system specifies positions in Cartesian coordinate systems for each state. There are one or more zones in each state, with slightly different projections in each state, with slightly different projections in each State Plane Zone. Multiple State Plane zones are used to limit distortion errors due to map projections. The Lambert Conformal Conic and the Tranverse Mercator are used in the State Plane coordinate System. The Lambert Conformal Conic is often used in states that have a long axis in the east-west direction, and v.v. for the Transverse Mercator.

Front 17

Can you define and describe the Universal Transverse Mercator coordinate system? What type of developable surface is used with a UTM projection? What are the UTM zones, where is the origin of the zone and how are negative coordinates avoided?

Back 17

The Universal Transverse Mercator Coordinate System is another standard coordinate, distinct from State Plane systems. It is a global coordinate system based on a Transverse Mercator projection. It divides the earth into zones that are 6 degrees wide in longitude and extend from 80 degrees south latitude to 84 degrees north latitude. They are numbered from 1-60 in an easterly direction, starting at 180 degrees West. Negative coordinates are avoided by creating false northings and false eastings.

Front 18

What is a datum transformation? How does it differ from a map projection?

Back 18

A datum transformation is a method or set of equations that allows the calculation of a point location in one datum based on coordinates expressed in a different datum. This is changing one 2-D representation to another 2-D representation. A map projection is a systematic rendering of features from a spheroid or ellipsoid representing the 3-D earth on a map surface. This is moving from 3-D to 2-D.

Front 19

Can you describe the Public Land Survey System? Is it a coordinate system? What is its main purpose?

Back 19

The Public Land Survey System is not a coordinate system. It is a standardized method for designating and describing land parcels. The United States needed a way to describe land in order to sell it and gain revenue. It is divided by north-south lines that run parallel to the prime meridian. The east-west lines run perpendicular to the prime meridian at six mile intervals. These lines form townships that were six miles square. Each township was divided into 36 sections that are approximately one mile on each side. Sections are remembered in a zigzag pattern.

Front 20

Why have so many digital spatial data been derived from hardcopy maps?

Back 20

Because for most of history, maps were hardcopy. Digital maps are being made but the process is relatively slow and painstaking.

Front 21

Can you describe three different types of generalizations?

Back 21

-Fused: multiples features may be grouped to form a larger feature
-Simplified: boundary or shape details are lost or “rounded off”
-Displaced: features may be offset to prevent overlap or to provide a standard distance between mapping symbols
-Omitted: Small features in a group may be omitted
-exaggerated: standard symbol sizes are often chosen and don’t always match the actual size of the entity.

Front 22

What are the most common map media? Why?

Back 22

A computer screen is the most common type of map media. Paper is the next one.

Front 23

Which map typically shows more detail- a large scale map or a small scale map? Can you give three reasons why?

Back 23

A large scale map typically shows more detail, because each feature is drawn larger and there is more opportunity to show variation in shape. Small errors in measurement are magnified by scale-factor and appear more on a small-scale map. Large-scale maps give more detail.

Front 24

What is snapping in the context of digitizing? What are undershoots and overshoots and why are they undesirable?

Back 24

Snapping is a process of automatically setting nearby points to have the same coordinates. Snapping relies on a snap tolerance or snap distance. Undershoots are lines that are supposed to be connected to another line but didn’t make it far and overshoots are lines that have crossed too far.

Front 25

What is the ‘common feature problem’ when digitizing and how might it be overcome?

Back 25

The common feature problem is when the same features appear differently on different maps. Features may appear differently on different maps due to blunders, coordinate errors, pointing errors or the maps were simply made with different purposes in mind. One way to fix this is to redraft the conflict areas. Another is to assume that there is a “master boundary” that has the highest accuracy, and the common features are established as a base that all other layers must match.

Front 26

Can you describe the general goal and process of map registration?

Back 26

Map registration if also referred to as coordinate transformation and is used to convert newly digitized data from the digitizer/scanner coordinate system. The input coordinate system is usually based on the digitizer or scanner’s assigned values. An image is scanned and the coordinates are recorded. These coordinates are then transferred onto an Earth-based map coordinate system.

Front 27

What are control points and where do they come from?

Back 27

Control points are used to transform digitized data from the digitizer or photo coordinate system to a map-projected coordinate system. Control points are different from other digitized features in that we know both the map projection coordinates as well as the projection coordinates. These two sets of coordinates are used to estimate the coefficients for the transformation equation. This equation is then used to convert coordinates from the digitizer system to the map projection.

Front 28

What is the root mean square error (RMSE) and how does it relate to a coordinate transformation?

Back 28

Root Mean Square Error is a statistical process that shows a summary of the difference between the true and predicted control point coordinates. It provides one index of transformation quality. The RMSE will usually be less than the true transformation error at a randomly selected point because we are actively minimizing the N and E residual errors when we statistically fit the transformation equations, but it is still a measure of accuracy and a lower RMSE generally indicates a more accurate affine transformation.

Front 29

Describe the general components of GNSS, including the three common segments and what they do.

Back 29

GNSS are satellite-based systems that give precise positioning information. The three components are: the satellite segment, a constellation of satellites circling the earth, the control segment, the tracking, communications, data gathering, integration and control facilities and the this is the user segment, the individuals with receivers.

Front 30

What are the basic principles behind GNSS positioning? What is range measurement and how does it help you locate yourself?

Back 30

GNSS is based on range distance measurements from multiple satellites to triangulate a location. Orbiting satellites transmit radio signals along with precise positioning and timing information. The current distance between a satellite and a receiver is a range measurement. A GNAA receiver combines multiple, simultaneous range measurements to estimate a location in near-real time.

Front 31

Describe the GNSS signals that are broadcast and the basic difference between carrier and coded signals?

Back 31

GNSS signals are based on radio signals sent by each satellite. Coded signals are sent and the time is measured between sending and receiving to gather positional information. In carrier signals, the waves are measured, making it more accurate.

Front 32

How many satellites must you measure to obtain a 3-dimensional position fix?

Back 32

Typically four satellites are required for a 3-dimensional fix, although a fix may be determined under some assumptions with data collection from 3 satellites over a short period of time.

Front 33

What are the main sources and relative magnitudes of uncertainty in GNSS positioning?

Back 33

The typical range error is 7.5 meters. The main sources include: satellite clock errors, receiver errors, satellite position error, the atmospheric and ionospheric effects as well as multi-path signals.

Front 34

How accurate is GNSS positioning? Be sure you specify a range, and describe under what conditions are at the high and low end of the range.

Back 34

GNSS data range in accuracy from sub-centimeter for the highest accuracy using carrier phase methods to tens of meters using real-time C/A positioning. Accuracies are highest when using high quality receiving systems in flat terrain with no buildings, trees or other structures. Accuracies improve when satellites are widely placed.

Front 35

What is a dilution of precision (DOP)? How does it affect GNSS position measurements?

Back 35

Positional Dilution of Precision is a measure of satellite spacing. It typically ranges from 2-10. Lower PDOPs are best. Ideally, the satellites form a tetrahedron with one satellite overhead with the others spaced at 120 degree angles along the horizon.

Front 36

Describe the basic principle behind differential positioning.

Back 36

Differential positioning is based on simultaneous measurements of GNSS signals to both a known, based location and at unknown roving stations. The small errors in range measurement may be calculated for each position measurement at the base station. These range errors may be applied in reverse for corresponding rover data, thereby improving the accuracy of position measurements.

Front 37

What is the difference between post-processed and real-time differential positioning? Can you list three pros and cons of each?

Back 37

In post-processed data, the rover and base-station data are brought to a computer and positions are determined after the fact. In real-time, corrections are communicated to the field through FM frequency.
Post-Processed Pros
-More accurate
-Does not require FM radio connection
-less expensive
Real-Time Pros
-More correct locations in the field
-Can use base stations provided by US government

Front 38

How is GNSS accuracy affected by the local terrain horizon? How is it affected by canopy cover or building obstructions? Why does positional accuracy change as these coordinates change?

Back 38

GNSS accuracy typically decreases as terrain become more varied or when canopy or buildings obstruct a portion of the sky. Positional accuracy decreases because sub-optimal constellations of satellites are more likely to be observed. Satellites are in closer proximity and measurements are less independent and hence to not reinforce each other to improve accuracy.

Front 39

What is WAAS? Is it better or worse than ground-based differential positioning?

Back 39

WAAS is Wide Angle Augmentation System, a real-time differential correction system designed to aid navigation in U.S. civil aviation. Correction factors are derived from a nationwide network of control stations and broadcast from a geostationary satellite located over the equator. The system is designed for aviation and related uses in North America.

Front 40

What are the main components of a database managing system?

Back 40

The main components of a database managing system are data items and attributes.

Front 41

What are the primary functions of a database management system?

Back 41

Database management systems are computer software tools that aid in the entry, organization, analysis, distribution and presentation of data

Front 42

Can you describe the difference between single and multiple user views?

Back 42

Single user views only allows one user to be using or changing the data base at one time whereas multiple user views allow multiple people to be using or changing the database information. Protocol must be created to resolve any differences that may come about with multiple users changing or adding information.

Front 43

What is a one-to-one relationship between tables? A many-to-one relationship?

Back 43

A one-to–one relationship among tables means that for every row in one table that is in some way matched to a row in another table, there is only one row in the second table that matches. A many-to-one relationship means that one row in a table may match many rows in another table. Note that by match, we do not mean completely match. Usually we are using a column in each table to match the tables; the rows are considered to match when the match column has the same value in both columns.

Front 44

Why have relational database structures proven so popular?

Back 44

Relational databases are so popular because it is more flexible than other designs. It is simple to understand and can accommodate a wide array of data types. It is not necessary to know in advance what types of queries are going to be run.

Front 45

What I the primary reason that hybrid database models are used for spatial data?

Back 45

They are stored in hybrid databases for rapid retrieval. Information is grouped for easier access. Pointers are used to link related lines or polygons and connect them to corresponding attribute data.

Front 46

Can you define and give examples of local, neighborhood and global spatial operations?

Back 46

Local spatial operations use only the data at one input location to determine the value at a corresponding output location.
Neighborhood spatial operations use data from both an input location plus nearby locations to determine the output value.
Global spatial operations use data values from the entire input layer to determine each output value.

Front 47

Describe selection operations

Back 47

These identify features that meet one to several conditions are criteria.

Front 48

Can you describe set and Boolean algebra?

Back 48

Set algebra uses the operations >,<,=, <>. It may not be applied to nominal data.
Boolean Algebra uses OR, AND, NOT.

Front 49

List and describe three different classification methods:

Back 49

Equal-interval class: divides up the scope evenly
Equal-area Class: boundaries are defined to place an equal proportion of the study area into each of a specified number of classes
Natural Breaks

Front 50

What is dissolve operation? What is it typically used for?

Back 50

Dissolve is a function used to combine similar features within a data later. It is needed prior to applying an area-based selection in spatial analysis. It helps get rid of unneeded boundaries.

Front 51

How are raster proximity functions different from vector proximity functions?

Back 51

Raster boundaries are measured from the cell center and are stair-step. Vector is much more variable.

Front 52

Describe the basic concept behind layer overlay

Back 52

Overlay is combining spatial and attribute layers form two or more spatial data layers.

Front 53

Why are output features in vector overlay typically set to a minimum dimensional order of the input features?

Back 53

The minimum dimension is chosen because to do otherwise courts with ambiguity. If two lower dimension features are coincident with higher-dimension features, it is unclear how the attricutes should be recorded in the resultant features. For example, if two points fal within a polygon, the polygon attributes may be unambiguously associated with each point. It is unclear at best to assign both sets of point attributes to an output polygon.

Front 54

Why is the sliver problem in vector overlay? How might this problem be resolved?

Back 54

Created when two maps have different boundaries. Can reduce them by identifying a common boundary and using it for all maps, identifying and removing manually and setting a snap distance.