I. GIS Building Blocks
First Course in GIS Part 1: Land Base and General Information about GIS
I. GIS Building Blocks
II. GIS versus Gee, I guess.
III. Land Base
IV. Satellite Imagery, its use in our industry
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2. A Georeferenced Information System(GIS) must have a Geographic base to reference information by. All information in a GIS can be related by coordinates that describe the location of the data object. Thus the information is geographically referenced or georeferenced. The Geographic base is referred to in our industry as Land Base.
http://www.compassusa.com/Landbase.htm
3. You may have heard of "georeferenced" before. Almost all the maps we produce at Compass are positionally correct and georeferenced. That is to say, the Microstation coordinates you can read in a .dgn file are the actual coordinates in some standard coordinate system.
4. Map names are also georeferenced. The geocoded name refers to the georeferenced corner coordinates. The lower lefthand corner of the map frame is the location reference for the name. Typically in a six digit map name, as three digits, dash, three digits are chosen to represent the corner coordinates. For example a map whose lower left hand coordinates are xy= 390000,1406000 would have a name like 390-406. The maps are located on even coordinates so that the trailing zeros may be ignored. Additionally, all the "y" coordinates are assumed to be in the 1 millions so the leading 1 is dropped. If that were not the case, to prevent repeats, seven numbers can be used. In this example the seven digit map number would be 390-1406. Sometimes the grid is made up of very small maps, because the scale is large like 1"=50 ft or 1cm = 10m. Then an eight digit map number may be used. The maps we did for a job in India had eight digit map numbers like 39005025, the maps were .5 km x .75 km in size. The name was pulled out of the coordinates as shown by the bold numbers: xy = 639000,3450250. The italic numbers were common to all the map names.
5. Geocoding may also mean an item such as an address is linked to its coordinates in a relational database. Because they related in this way, an address may be used to determine what map the site is located on, and coordinates may be used to find the nearest addresses to a site. When a customer, who is linked to his address by way of a customer ID number, (and can be identified by his coordinates), moves, his address changes when his coordinates are changed. His other attributes are linked to a customer ID number, so they move when he moves. If the linkage is broken, the customer is without coordinates and lost in the system. A exception report finds these customers and they may be manually restored to the database with coordinates. The actions of scrubbing and parsing are part of what the database department do and beyond the scope of this document. It should be obvious that because of this linkage, any of the customer items may easily be made to include all or part of the coordinates. An example might be Bart 39005025 Simpson. From the listed name, we could find out a lot about Mr. Simpson. We could also tell him apart from Bart 39255100 Simpson, a customer in a adjacent sector. Naming Schemes are another topic we will not approach in depth. One caveat is included however; never use the client social security number in any client identifier. This information may be relationally stored for verification purposes, (preferably off line) but it should not be made public. See PHRACK, for some ideas about what is and isn't secure.
II. GIS versus Gee, I guess._____________________________________ Back to the top
A geographic information system (GIS) that is useful for our industry consists of a relational database and an associated graphical interface. This is different from the flat file, cad drawing map retrieval type system you may be more familiar with.
A properly constructed GIS includes positionally correct mapping that fits together and supports all types of graphical queries. An example would be a query to determine the length of any routing, regardless of how many maps were involved. Features are generated from the data base, allowing logical queries. An example would be "select customer where street = 'Pine' and custname like 'Kneedle'. The connection between the map (the graphical interface) and the relational database allows graphical queries such as select all customer 'Kneedle' within 5280 feet of the center of hub 'B' (of course this might be done by drawing a circle in the hub and typing cust=Kneedle). Customer information is portable; if the customer moves, his coordinates change, but the information is otherwise intact. And, if the customer moves, the element in the map moves too. That's GIS.
A flat file, cad drawing map retrieval system utilizes "maps" which are related by a key map. Customer information is found somewhere else; possibly in a spreadsheet. Hardwired routines allow you to query based on what the programmer thought you would be doing. Data entry variations such as "L'ville Suwanee Road" and "Lawrenceville Suw Rd" guarantee much of your data will be lost most of the time. Updates to the maps have no effect on the data. Updates to the data have no effect on the maps. The maps probably have "0,0" as the lower left corner coordinate. Any work involving more than one map involves guess work. And by coincidence, nearly all your work involves more than one map. When a service request comes in, if you can locate the customer in the database, and are able to find him on your map, (if the last update added his street) you might be able to help him. That's Gee, I guess.
III. Land Base:____________________________________________ Back to the top
1. Map Projections
2. The Right Projection
3. What Our Land Base is Supposed to Do.
References:
1. The Geographers Craft Project, Dept. of Geography, The University of Texas at Austin,
copyright 1995 Peter H. Dana
2. Digital Map Users Guide for Microstation, American Digital Cartography Inc (ADCi)
3. Zone 27.doc, Ashtech: GPPS documentation, Ashtech, Inc. Sunnyvale Calif.
1. Map Projections________________________________________ back to III. Land Base
a. The following map projections/coordinate systems will be described.
i. State Plane Coordinate System(s)
A. Transverse Mercator Projection (ref 1)
B. Lambert Conformal Conic (ref 1)
ii. Universal Transverse Mercator (UTM)
II.1.a.i The State Plane Coordinate Systems (taken from ref 1)
In the United States, The State Plane System was developed in the 1930s and was based on the North American Datum 1927 (NAD27). NAD27 coordinates are based on the foot. While the NAD27 system has been superseded by the NAD83 system, NAD27 coordinates are still widely used. When coordinates are given, the datum should always be specified.
State plane systems were developed to provide local reference systems that were on a national datum. Lambert Conformal Conic projections are used for rectangular zones with a larger east-west than north-south extent. Transverse Mercator projections are used to define zones with a larger north-south extent.
STATE PLANE ZONE CODES(examples)(ref 3.)
CODE STATE ZONE PROJECTION 0600 Connecticut Lambert 2001 Massachusetts Mainland Lambert 2002 Massachusetts Island Lambert 2900 New Jersey Transverse Mercator 3101 New York East Transverse Mercator 3102 New York Central Transverse Mercator 3103 New York West Transverse Mercator 3104 New York Long Island Lambert
II.1.a.i.A Transverse Mercator Projection (ref 1)_______________ back to Map Projections
The Transverse Mercator projection is one of the two projections used in State Plane Coordinate systems. In these systems the central meridian lies in the center of the state plane zone. This projection is a cylindrical projection. This means that the latitude and longitude are projected onto a cylinder whose surface touches the earth at and only at the central meridian.
II.1.a.i.B Lambert Conformal Conic projection (ref 1)__________ back to Map Projections
This projection utilizes a cone whose surface touches the earth at two parallels of latitude. These parallels are called standard parallels.
Used by the USGS for many 7.5 minute and 15 minute topographic maps and for the State Base Map series, this projection is also used to show a region that is mostly east-west in extent. This is one of the most widely used projections in the U.S. today. For the USGS Base Map series for the conterminous 48 States, the standard parallels are 33 N and 45 N. Maximum scale error for the map of 48 states is 2� %.
II.1.a.ii. Universal Transverse Mercator projection (taken from ref 2.) back to Map Projections
The Universal Transverse Mercator system (UTM) is an international metric coordinate system based on a Transverse Mercator projection. This system divides the world into sixty north-south zones, each of which is 6 degrees of longitude wide. Beginning with zone 1 between 180 and 174 degrees west, zone numbers increase in the easterly direction. For example the NY, NJ, CT, MA area falls in zones 17, 18, 19.
Maps in the UTM system adjacent to one another in the same zone will "match up" perfectly. Maps falling on the edge of a zone will not match the maps on the edge of an adjoining zone, however, because the characteristics of the projection change as the zone boundary is crossed.
One solution to mapping the NY, NJ, CT, MA area would be to map everything as if it all fell in UTM zone 18. Absolute accuracy would suffer, but the solution is a simple one. Of course, calculated coordinates would fall outside the standard range for UTM coordinates. This may in turn have unforseen effects on various software packages.
2. The Right Projection; an example using a large area____________ back to III. Land Base
Comparison
For the NY, NJ, CT, MA area, using standard parallels 40'15 N and 43'15 N, A Lambert Conformal Conic projection would be expected to have a maximum scale error of less than 1%. In comparison, the use of one UTM zone for the whole area would be expected to have a maximum scale error of more than 2%. Moreover, in the UTM case a much greater area would be subject to scale error of more than 1%
Addressing the Considerations:
i. Edge matching.
In order for edges to match, the various State Plane Coordinate systems mentioned in the four states example cannot be used. There are 8 separate grids with 8 different origins, based on two very different projections.
There are only two reasonable ways to assure edge matching from one end of the project to another. One is to use the UTM zone 18 for the entire project. The other is to use a Lambert Conformal Conic (LCC) projection with parallels chosen to minimize error.
ii. Accuracy
Maps made using the single UTM zone could be expected to include scale errors greater than one percent over more than half the project area.
Maps made using the LCC projection could be expected to include scale errors of less than one percent at all points within the project. Moreover, These maxima would occur only at the extreme south tip of New Jersey and at the Northern border of New York. More than half the project area would have errors less than � of one percent.
iii. Use of a Standard Product
I have spoken with various vendors and found that neither the one UTM or the LCC with parallels chosen to minimize error present any difficulty, nor is one more difficult than the other.
Links to Other Sources of Information:
<1.> Corps of Engrs Topographic Engineering Center
<2.> Links to GIS companies, associations,
and government web pages as well as sources of software,
back to III. Land Base
The following things are intended to be very accurate:
The following things are intended to be schematic in nature. This is intended to make the drawing more legible.
The following things are shown different than their scale size in order to show them legibly or to fit them on the paper. They include:
IV. Satellite Imagery, its use in design __________________________ Back to the top
We purchase georeferenced digital orthophotography. Various sources are available. They include:
SPIN-2 Resolutions 2 meter, 10 meter KVH1000, TK350
RSI Canada Resolutions 8-10 meters Radar Imaging
Other sources may become available in the future. These other sources depend on satellites that have not been launched yet.
Imagery from SPIN-2 is provided in tiff format. Spin-2 provides upper left corner coordinates and image size (pixels) as well. This is essential for calculating the lower left and upper right corner coordinates which Microstation uses.
An example of what we do, using the 2 meter product from SPIN-2:
We create accurate Landbase information from the Russian 2 meter satellite imagery. This Landbase information includes roads, railroads, rivers and lakes, and major buildings.
We use Microstation 95 in its native form without a third party design or a drafting package. We bring the imagery into Microstation 95 as a reference raster file. We then digitize the roads, railroads, rivers and lakes, and major buildings as per the project requirements.
Although local "tourist maps" may be available, they rarely provide the level of detail that can be obtained using satellite imagery. More importantly, the available mapping is almost never positionally correct. These local maps, while not to scale, give a good schematic representation of the road layout. They can be scanned in, digitized, and rubber sheeted to match monument points. Using the satellite imagery, the resulting map can be edited to produce a geocoded positionally correct product. This may be easier than relying solely on satellite imagery and increases confidence in the accuracy of the finished product.
Finally, our field people verify all the mapping by checking it in the field. They measure all distances critical to design, check for completeness of the road pattern, and make notes of everything the drafter will have to add before the map can be designed on.
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Mike Kretsch, P.E.
Mike Kretsch is a registered Professional Engineer in the State of Georgia. He has over 10 years experience in all aspects of Civil Engineering, plus four years experience in surveying. Other registration includes LSIT (surveying). Mike is a member of the National Society of Professional Engineers and a past Treasurer of the Gwinnett Chapter of the Georgia Society Professional Engineers.
Mike is currently employed by Compass Communications in Land Base Development and Technical Support. Duties include determining best land base for a given job, verifying accuracy of mapping, developing map base from satellite imagery, referencing mapping to monumented geodetic control, and technical support of database and GIS projects.
Mike has worked as a Principal Engineer for Gwinnett County Department of Transportation, and Gwinnett County Department of Public Utilities. (5 years Gwinnett County). Prior to that he worked for Precision Planning Inc. as Project Engineer. This included hydrology, storm water management, road, highway, bridge, water system, wastewater system, landfill, waste water treatment plant design, water treatment plant design, etc.
Prior to that Mike worked in surveying, as engineer, crew chief, and as instrument man. While at Ga Tech, he worked for the Corps of Engineers, and Ga. Dept of Natural Resources.
Mike received his Bachelor of Civil Engineering from Georgia Tech in 1983.
Mikes hobbies include astronomy, Tae kwon do, practical combat pistol competition, bicycling,
guitar and graphic arts.
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