The way maps and other data have been stored or filed as layers of information in a GIS makes it possible to perform complex analyses.

Information retrieval

What do you know about the swampy area at the end of your street? With a GIS you can "point" at a location, object, or area on the screen and retrieve recorded information about it from offscreen files (fig. 16). Using scanned aerial photographs as a visual guide, you can ask a GIS about the geology or hydrology of the area or even about how close a swamp is to the end of a street. This type of analysis allows you to draw conclusions about the swamp's environmental sensitivity.

Topological modeling

Have there ever been gas stations or factories that operated next to the swamp? Were any of these uphill from and within 2 miles of the swamp? A GIS can recognize and analyze the spatial relationships among mapped phenomena. Conditions of adjacency (what is next to what), containment (what is enclosed by what), and proximity (how close something is to something else) can be determined with a GIS (fig. 17).

Networks

When nutrients from farmland are running off into streams, it is important to know in which direction the streams flow and which streams empty into other streams. This is done by using a linear network. It allows the computer to determine how the nutrients are transported downstream. Additional information on water volume and speed throughout the spatial network can help the GIS determine how long it will take the nutrients to travel downstream (figs. 18a and b).

Overlay

Using maps of wetlands, slopes, streams, land use, and soils (figs. 19a-f), the GIS might produce a new map layer or overlay that ranks the wetlands according to their relative sensitivity to damage from nutrient runoff.

Data output

A critical component of a GIS is its ability to produce graphics on the screen or on paper to convey the results of analyses to the people who make decisions about resources. Wall maps, Internet-ready maps, interactive maps, and other graphics can be generated, allowing the decisionmakers to visualize and thereby understand the results of analyses or simulations of potential events (fig. 20).

Framework for cooperation

The use of a GIS can encourage cooperation and communication among the organizations involved in environmental protection, planning, and resource management. The collection of data for a GIS is costly. Data collection can require very specialized computer equipment and technical expertise.

Standard data formats ease the exchange of digital information among users of different systems. Standardization helps to stretch data collection funds further by allowing data sharing, and, in many cases, gives users access to data that they could not otherwise collect for economic or technical reasons. Organizations such as the University Consortium for Geographic Information Science (www.ucgis.org) and the Federal Geographic Data Committee (www.fgdc.gov) seek to encourage standardization efforts.

For more information

Good places to learn more about GIS technology and methods include the geography department of your local university, the GIS site at www.gis.com, your county planning department, your state department of natural resources, or a USGS Earth Science Information Center (ESIC). To locate your nearest ESIC, call 1-888-ASK-USGS, visit ASK-USGS web site, or visit www.usgs.gov.

Source: http://egsc.usgs.gov/isb/pubs/gis_poster/

 Links

Vector Topology

Topology is the spatial relationships between geographic features. It is not to be confused with topography, the form of the land.
 

1. The Components of Topology

Topology has three fundamental  components:

a. Connectivity:
Arcs are connected to others (at nodes). This identifies possible routes and networks, such as rivers and roads, via the lists of arcs and nodes in the database.

b.  Containment:
An enclosed polygon has a measurable area; lists of arcs define boundaries and closed areas.

c.  Contiguity:
The adjacency of polygons can be determined by shared arcs.

Table 6-1

Polygon Topology: Area		Node Topology: connectivity		Arc Topology: contiguity
Polygon	Arcs	Node	Arcs	Arc	Left & Right Polygons
A B C D	a1, a2, a3 a2, a5, a6 a3, a4, a5 a1, a4, a6	1 2 3 4	a1, a2, a6 a2, a3, a5 a1, a3, a4 a4, a5, a6	a1 a2 a3 a4 a5 a6	A  D A  B A  C C  D B  C B  D

 These are fundamental to GIS analysis and queries, for example:

a. From point A, how can I get to point B using the city road system?
b. What is the area of the combined areas of all residential housing?
c. Which residential areas are next to city parks?

See also this website: http://www.innovativegis.com/education/primer/concepts.html

Diagram explanation:

• Polygon A is bounded by arcs a1, a2, a3  etc..
• Polygon D is known as the 'External or Universe Polygon' which describes all areas OUTSIDE the polygons on your map.  It is necessary for reasons of contiguity, all arcs are bounded by two polygons. It would help answer, for example, if you were working in a park: how many areas are adjacent to the park boundary?

• Node 1 is connected by arcs a1, a2, a6  etc..
• Each node is connected by at least (and usually) three arcs

• Arc a1 is bounded either side by polygons A and D  etc..
• Note: arcs are usually created with a direction, i.e. a 'from' node and a 'to' node. The actual direction may be significant, for example in stream flow, but arbitrary in others, e.g. most roads. Direction determines which polygons are 'left' and 'right'.

GIS vector data can be either constructed with topology (topological data) or without topology : 'spaghetti' data  (see below)

2.  Spaghetti versus Topological data

a. 'Simple' spaghetti data

• Points:  have  x and y coordinates.
• Lines (arcs): Strings of x, y vertices.
• Polygons: Closed set of coordinates.
• But there is NO spatial relationship between these features:

• Arcs may not necessarily join and Polygons may not close to form areas.
• Intersections may not have nodes where two arcs cross.
• Adjacent digitized polygons may overlap or underlap (leaving an empty wedge).
• Arcs may consist of many broken segments.

b. 'Complex' topological data

• Points: are polygons of zero area and length.
• Lines (arcs): start and end at nodes.
• Polygons: given by sets of connected arcs and an interior label point.

• Lower total number of arcs in a database.
• Adjacent polygons do not enclose overlap wedges or slivers.
• Cleaner map output (more evident when you zoom in or magnify).

This is the raw data. It must be 'cleaned and built' for GIS. There are unacceptable dangling arcs, nodes, and missing intersections.	The data after "clean & build": there are nodes at all intersections, and no dangling arcs.

3. Creation of Topology: 'Clean & Build'

a. Node types

	EXAMPLE	DEFINITION	ACCEPTABLE
NORMAL NODES		At an intersection of 3 or more arcs	Always
DANGLING NODES		At the end of an arc	Arcs (Not polygons)  e.g. roads, streams
PSEUDO NODES		Between 2 arcs	Island polygons, attribute change

b. Arcs

• Nodes are required at all arc intersections.
• Dangling arcs can be accepted if the "dangle tolerances" are set.
• e.g. if tolerance = 5 metres, an arc < 5 is a dangle (error), an arc > 5m is a legitimate arc.

c. Cleaning (moves nodes/arcs)

• Removes unacceptable dangling arcs and nodes.
• Joins missing arcs segments (within a special distance).
• Removes unnecessary pseudo nodes.
• Adds nodes to all intersections.
• Label points are added to polygons.

 d. Building topology

• Does not move any features but 'cements' them into place.
• Creates a Feature Attribute Table.
• Builds (again) after new edits including,
• addition or removal of arcs and points;
• addition or removal of attribute items.

4. Review

1. Name & describe the three components of topology.
2. There is no spatial relationship between points, lines and polygons in spaghetti data. True or false?
3. When are dangling nodes acceptable?
4. Define normal node, dangling node & pseudo node.
5. Name four things that 'clean' does.

Source: http://www.gis.unbc.ca/courses/geog300/lectures/lect20/index.php

 Links

Maps: Projections and Datums

Where did you say you were calling from?

• The round earth on a flat map = "a projection"
• imagine putting a strong light bulb in the globe and then holding flat piece of paper around it.
  
• answer? Flat, wrapped cylinder, wrapped cone, cut up into wedges.
• Some common ones (ask what the poles/greenland will look like)

•  cylindrical (Mercator)
• Azimuthal (for poles)
• Conic (one latitude is tangent, called the standard parallel)
• Poly-conic (multiple latitudes tangent)

Here's a comparison of some, with some projection views.

(excellent source for more information here)

Projections create distortion

ArcGIS Help on projections says

All map projections distort shape, area, distance or direction to some extent. The impact of this distortion on your work depends on what you will be using your map for, and its scale:

The map has different names, depending on what you hold constant:

1. equal area--homolographic

if you put a dime on a map, it'll always cover the same area, but shapes and angles are distorted
2. shape - conformal

at a point, relative local anles are preserved
meridians intersect parallels at right angles
areas enlarged or reduced
3. Scale - equidistant

equidistant between two points and rest of map only, or along meridians
4. directional - azimuthal

all rhumblines (lines of constant direction) are straight

Create a new blank arcmap document, then open VA_counties.shp from your GIS\demo\intro folder and change the coordinate system of the frame to

• state plane=polyconic
• utm, grid zone 17
• equal area
• decimal degrees (geographic)

Geoid and reference elipsoids

The earth is an oblate spheroid with the minor axis 1/300th shorter than major axis but the earth also has an irregular undulating surface that varies by +/- 100m from the oblate spheroid.  So the geoid is the approximation for shape of earth at "sea level" that takes into account gravitational and rotational inconsistencies.

this irregular shape is approximated by "elispoids" with a major and minor axis that fit particular parts of the globe better than others.  In the US, the North American Datum of 1927 (NAD 27) uses the Clarke66 (that's 1866) elipsoid, named for British geodisist Alexander Ross Clarke.  He measured the meridian arcs in Europe, Russia, India, S. Africa and Peru (with chains and surveying instruments).  His radii are 6,378,206.4 m and 6,356,583.8 m for the equatorial and polar axes, respectively. In ArcGIS, open the Data Frame Properties/coordinate system box, and choose the "predefined-geographic-spheroid based-clarke66" coordinate system and click "modify..." to see its parameters.

Datums fix the zeros

A reference elipsoid must have a zero point somewhere and the mean sea level must be established from the geoid.  These are fixed for a particular UTM zone, for example, or standard parallel.

Satellite data are currently being gathered and fitted to world-centered elipsoids (e.g., WRS80) that do a better job for the entire surface of the earth as a whole, but less well than the hundred year old regional elipsoids at any given location.

And so there's a problem.  Some USGS data come from NAD27 maps (Clarke66 elipsoid) while others, like the microsoft terraserver use NAD83 (WRS80 elipsoid).....
look at datum shift (for lat long) for NAD27 to NAD83 on BV quad
Want to see it?
open demo\projections\projections_and_datums.mxd

So where are you????

The datum shift includes
initial point change (to center of earth from reference elipsoid matched to earth's surface)
change in length of elipsoid axes
ground survey network changes (local deviations, distortions, errors, tectonics)
Plate tectonics?? (opening is 15 cm/year * 60 years....)

UTM (Universal Transverse Mercator)
computers eat it for breakfast.
it reads like cartesian coordinates

here's a picture of how it works (by Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of
Colorado at Boulder)

Grid zones are 6 degrees wide with a central meridian
They start at 180 degrees west of Greenwhich and go east.  In Lexington, we're in grid zone 17, which has 81 degrees W as the central meridian.
US Grid Zones and World Grid zones

ArcMap: For UTM / Geographic comparison,

• Open projections_and_datums.mxd from the Demo\projections folder

• Move the dataframe projection from one grid zone to another (eg. zone 17 to zone 18, then to zone 10 !)

Source: (http://geology.wlu.edu/harbor/geol260/lecture_notes/notes_maps1_new.html)

 Links

Martian Landscapes: Linear Features, Volcanoes, Impact Craters, Channels; Exotic Terrains Part-2 - Application

A special type of volcano on Mars has recently been proposed to be caused by escaping gas, fluid, and mud. These 'mud volcanoes' have their counterparts on Earth. On Mars they are observed in the thousands in Acidalia Planitia in the northern Plains. Their importance is that the fluid is likely water and hence such features are more likely to harbor at least single-cell life. Here are two examples:

Among martian features believed volcanic in nature are linear ridges similar to the wrinkle ridges found in lunar maria. Here is a topographic map made from MGS MOLA measurements that includes (in the purple) these ridges and shows the diversity of other landforms.

Another example of what is interpreted as wrinkle ridges is this:

In contrast to the volcanoes described above, which are upward conical prominences, are the downward indentations or craters that can be either volcanic or impact. Both are typical of martian terrains. Extensive impact cratering was observed by Mariner 4, which sent back the first ever images taken of another planet's surface (one of these images is seen below (top) when this probe approached to within 9800 km (6086 miles). As imaged the next year by Mariner 6, the Sinus Sabeus region of the southern highlands (bottom scene) preserves typical impact craters in the ancient terrain that apparently has not been extensively resurfaced by lavas. Note that none of the larger craters in this view have central peaks.

Mariner 9 and the Vikings confirmed that a large fraction of the (older) martian surface, mainly in the southern hemisphere, remains heavily cratered. This is evident in this sketch drawing from Mutch et al., The Geology of Mars, 1976 in which all craters larger than 15 km are positioned.

A recent study made by Dr. Herbert Frey of NASA Goddard - assisted by his teen age daughter Erin - has led to a map of the distribution of large surface-visible plus now buried impact structures that nevertheless show circular surface manifestations. The latter have been located using the MOLA laser altimetry data.

One can argue that this landscape has many similarities to the still cratered Earth in its early stages before extensive water had collected into major oceans. Likewise, buried impact structures can be discerned on the lunar surface. These have since been covered by lunar ejecta. This may mean that the martian Highlands surface is also covered by ejecta deposits that spread over older craters.

Some of the martian impact structures retain well-preserved ejecta blankets that display prominent lobes, such as seen here around the crater Yuty. The ejecta was probably fluidized by vaporization of carbon dioxide-rich ice lying just beneath the surface.

One type of impact crater is different from those on the Moon, Mercury and Venus in that the edge of the ejecta blanket has a steep scarp, evident in the Viking image below, or even a peripheral rise called a rampart. This type is called a pedestal crater.

On Mars many of the younger craters still preserve their ejecta blankets, as exemplified here:

This next crater is small, young, and shows most of the same features as do terrestrial craters. Located in Terra Meridiani, this crater is 2.6 km wide (1.6 miles; rim to rim), has at least 1 nested slump zone in its interior and a distinct exterior ejecta blanket, and has exposed what appears to be internal layering of the martian surface units. The image was made by the Mars Global Surveyor.

This type of central (interior) layering, almost certainly sedimentary (see pages 19-13a and 19-13b) also appears in the 2.3 km (1.5 mile) wide Schiaparelli crater in the Chrysae Basin, seen below. The layering appears horizontal:

These observations of sedimentary-like crater interior floors and walls (layering is also discussed on the next page) seem rather mysterious. On Earth, craters that still retain their original rims (almost?) never show the bedrock below the final crater excavation wall. Yet this is common in martian craters with initial walls intact. Martian planetologists have suggested removal by erosion (they mean almost certainly wind erosion). There may be an alternate cause: the lower martian gravity allow nearly complete escape during crater formation of the bulk of the ejecta; the floor remains exposed because in the smaller craters slumping has not destroyed the walls.

This small crater (below) shows a distinct pattern of dark rays. Because martian winds are continually altering the surface, both removing and covering up debris, the crater (and those above with lighter-toned rays) can be young - age estimates have ranged between a few thousand and a few million years.

This rayed crater looks fresh. Experience on Earth indicates that impacts occur rather often in terms of a human time frame. A new crater was produced on Mars during the operational period of the Mars Global Surveyor. This before-and-after image pair shows the appearance of dark rays around an area which contains a small hole not there on the earlier date:

Because of several factors, some martian craters appear as faint rings rather than topographic features raised above the surface. These have been called "ghost" or "stealth" craters. They represent some combination of burial by crater ejecta, wind erosion, dust cover, and ice cover. Here is an example of this last type:

There is evidence that the number of observed impact craters on Mars is less than would be expected if the recent activities (dust transport and deposition, ice relocation, etc.) had not buried the smaller ones. The wind, however, is capable of exhuming such craters, as displayed in this image which also shows the exposed craters to contain some signs of filling by sediment, now revealed as faint layers.

Not all impact craters are circular or slightly elliptical. Strongly elongate craters are found on the Moon. A few such distorted craters are present on Mars, such as the one shown below. The usual explanation is that the impacting body comes onto the surface at a very low or grazing angle, scouring out the surface material as it proceeds forward:

Large, young impact craters are few but conspicuous. Galle Crater is 220 km (138 miles) wide and retains its original rim:

As with the Moon, Mars has a few craters so large that they can be called impact basins. By far the biggest is the Borealis Basin (also known as Vastitus Borealis). Mars geologists (starting with George McGill and Stephen Squyres) have postulated that this feature (which has dimensions of 10,600 by 8500 km) was produced by a glancing collision with an asteroidal body that may have been as much as 1600 km in diameter. This impact, which peeled off at least 3 km of martian surface, may have occurred as early as 4 billion years ago. It accounts for the generally lower topography of the northern half of Mars (see page 19-10), evident in this topographic map (blue low; reds high):

Papers from a group at MIT in a June 2008 issue of the journal Science offer strong evidence for the existence of the Borealis Basin; as is often the case, there is a vocal group of doubters. But, the impact hypothesis is a plausible explanation for the dichotomy in martian topography: the distinctly lower top 40% is readily explained as caused by the stripping of crust from an oblique impact. In the next figure, the Borealis Basin (bluish area in upper right panel) is compared with the Hellas Basin on Mars and the Aitken Basin on the Moon:

The largest well-defined impact basin on Mars, and second in the Solar System only to the Aitken basin on the Moon, is the Hellas Basin in the southern Highlands. Its diameter is about 2100 km (1300 miles), its depth is almost 9 km (6 miles) and its rim exceeds 1.5 km (1 mile). In this view the Basin appears to have no significant landforms within it.

To emphasize the size of this structure: If all material excavated from it were to be spread evenly over the 48 continental United States, a layer of debris some 3.5 km (2 miles) thick would accrue. Below is an enlargement of the map covering this structure.

The floor of Hellas actually shows diverse landforms (mostly of low relief), some of which appear volcanic in origin; if so this would imply that the basin filled with melt soon after the impact event, which may have been relatively recent.

Another impact structure is the Argyre Basin (600 km; 390 miles diameter), seen in this Viking view:

Source: http://rst.gsfc.nasa.gov