GIS Fundamentals PDF

25 Bolstad, Paul & Steven Manson (2022). GIS Fundamentals: A First Text on Geographic Information Systems. University of Minnesota - Twin Cities. Eider Press. White Bear Lake....

25 Bolstad, Paul & Steven Manson (2022). GIS Fundamentals: A First Text on Geographic Information Systems. University of Minnesota - Twin Cities. Eider Press. White Bear Lake. Minnesota. 732 p. 2 Data Models Introduction Oa1a in a G!S represen1 a simplified ,iew ersbip. and land use. Smaller polygons and of physical emiries - the roads. mouolains. unrecorded characteristics such as value a.re accident locations, orotJ1er features we care not included in this representation. about Our d~1a record spa1ial loca1ion and Adata set's spa1ial de1ail and essential nonsparial properties. characteristics are subjecti\·ely chosen by the Each enlity is represented by a spatio/ da1a developer. The de1ail r' it; 1tpn«ntcd by l\ s.p:ui:i.l bjctl inn. GlS. Herc, fakc-s (d:u-k IIIJ'C'M in dtc pho.t()o grnph) and otbc:r land eo,-cr types are rcprcacoted by polygon, in 1:be data layen on 1bc rig!Jt. 26 GIS Fundamentals Real world Data model J ·,. / Data strucwre " x 12 y., t -.. ' 5.8 8.9 3.o 7.l ID Meo Type Macihine l 16.3 PUB code 2 7.9 PEM 3 12 1 8 u 1001 1101 00110110 4 10 l PVS 10110100 Flautt 2-2: Lends ,o( abs:IJ':lctiou in 1hc r¢p~sciuation o( spatjaJ C11ti1i«. TI1c l'Cal world is rcp~tcd in rucccui\ cly 111er. and a fla:utt 2..J: Spatiaidllta 11tt often stOttd llU q>Mlte roads daca layer. The roads layer contains tltc:matie: layc:n, with objec,s grouped b:u.c:d on :. only roads data. including the location and 5Ct of prope11i«, e.g.. w~tcr, ronds. o:r faod cover, properties of roads in the analysis area. or some otbc:ragrced-upon set. lnfonnation on soils, poLi1ical boundaries. and elevation are contained in their respec-  Chapter 2: Data Models 27 Coordinate Systems Coorrli11mes are used 10 define 1be spa- (3.2,15) tial location and extent ofgeographic objecis 15 (8.4.10) (Figure 2-4). A coordinate most often con- SiSIS or a pair or triple, or nwnbers 1lul1 spec- A""l'rbulU ify locotion iu rela1ion to an origin. The y 10 , , , 13'7 coordinates quantify the dislance from the ,:; J~ftt'. v.~ origin wbeu measured along standard direc· tions. Spatial data in a GIS most often use ·~ ,, IC-,, ! iool--lM (13.7 1) coordinate p.1irs, Xand Y, in a Canesian 5 coordinate sys1em, named after Rene Des- cartes. the system's origiuator. These pairs define data on a planar. two-dimensional (5 8.2.l) surface. The two-dimensional surface is usu- 0.0\ 5 10 15 ally based up,oo standardized metliods of origin x mapping 1be ground surface lo a flal map surface (discussed in Chapier 3). Typically. Fi&ot t 2-4: Coordinate and attn"butc data arc a11ribu1e daia complemeo111te coordina1e used 10 rcptescm c:iuitics. da1a for carto:gr.iphic objecis. These auribme data record the non-spa rial components of rut object such as a name. color. pH. or cash axes (right angle. or 90°), fomting a plane value. Keys. Labels. or 01her indexes are used (figure 2-5). We specify a Y-axis. usually so tl1at lhe coordinate and attribute data may aligned at or dose 10 a nortlt-south direction. be viewed. re la1ed. and manipula1ed and an X·axis. usually align,ed at or near an east-west directiOtL The Y-axis is often 1oge1her. referred to as a ,,or1M11g ,1.ris and values Planar. 1wo-dimensional(2-D) Cartesian increase upwards in a grid north direction. coordinate systems define two 011hogo110/ The x-axis is often referred 10 as an eosii11g a:ris with values increasing 10 the right 2-D Cartesian Coordinate Systems y 0 v 0 M 0 N -0 0 0 10 20 30 40 50 x Flan~ l -5: ~ 2-D coordinate system dcfio" X and Y a.,es (left paocl i.u figw-c abo,-c). and ~i(y coordi· 1uuc loc:uioi11s by tbcsc X-Y pdlN. Cootdiiutc nah.1cs incn:-asc ll) ria)uwntd (X) :ud upw.t1td (Y) du'C'CliOlU, :md lines of consll\UI X Or Y ,·:tlucs mny be 11,kd 10 ~id in loc:uion Oii innpt (tight. abo,"C:). 28 GIS Fundamentals We must be careful when making mea- frnm -90 at 1he South Pole to 90 at 1he North surements on our Oat. 2·0 surface. These Pole. Lines of constant la1itude are called measuremen1s un:avoidably diston relative parallels (Figure 2·6). locations, because the Earth's true surfac-e Geographic coordimues do 001 fonn a approxima1es a sphere and hence is curvoo. Cartesian sys1em becaiise the meridians We can keep errors small by limi1ing 1he c,onverge. A Cartesian system defines lines area over which we use our flat map. As the on a righi-:1ngJe, planar grid. Geographic mapped area ge1s larger. !he error increases coordina1es ocC11r on a curved surface. and to amoun1s we usually can'1 ignore. Specific the longitudinal lines cross at tbe poles. conversions from 3-0 to 2-0 coordinate sys- This convergence means the distance tems and methods for managing dis ton ion in spaimed by a degree of loogimde varies Oat map systems ru-e discussed in Chap1er 3. from approximately l 11.3 kilorroeters at the Equator. to Okilometers at the poles. In Coordinates on a Sphere conltllSL 1he ground distance for a degree ofla1itude varies only slig)itly. from 110.6 When we map over larger areas or when kilomelers at the Equa1or to 111.7 kilome- we need lhe highest precision and accuracy, 1ers a, the poles. The slight difference wilh we often use a three-dimensional. sphel'ical la1in1de is due to a non-spherical Earth. coordinate sysrem. H_jpparcbus. a Greek something we'll describe a bit later. mathematician of the 2nd century 8.C.. was among lhe first IC) specify ioca1ions on lhe Convergenceca11ses distortin because a Earth using angular measurements on a degree of latirude spans a grealer distance sphere. Our familfar Gecgn,pltic C(J()rt/imue near 1he poles than a degree of ingimde. System uses two angles of rotation. the lon- For example. "circles" with a fixed radius in gin1de 0,). and !he la1imde ($). and a radius. geographjc units, such as 5°, are not circles R. to specify locauions on Earth (Figure 2-6). 011 tlie st1rfate of the globe, witli di~tortion The longitude measures east.west distances grea1es1 at the poles (Figure 2·7. left). They around the polar Eanb axis. Zero is se1 for a may appear as c ircles when the Earth's sur. line th.11 passes through England, and 1he face is..unrolloo.. and ploned 011 a Oat map distance angle is positive eastward and nega· (Figure 2-7. right). b1111rea1ing spnerical tive wes1ward (Figure 2·6). Lines of equal coordinates (la1imdesllongitudes) as Carte- lougirude are call~d meddlans. and are ori- sian coordfoates creales an inherently dis- ented north-south_The zero longitude. aJso 1orted map. NOie lhe distorted sbape of known as the Pn·me Men·d;m, or the G,.ee,,.. A.LHarctica in Figure 2-7. right. wich A1eridio11. was first specifioo 1brough Because the spherical sys1cm for geo- the Royal Greenwich Obse!\'lltory because grapltic coordina1es is noo-Canesian. pla- they had amassed lhe best early measure· nar fommlas for area, distance, angles, and me.uts. but the Prime Meridian bas now olher geometric properties used in a Cane- shifted about I02 meters (335 feet) east of sian coordinate system shou.ld not be used the Greeuwicb Oll>servatory as measure- with geographic coordinates. Are.as are meuts improved. ·1ec1onic pla1es shiftoo. and usual!y calcula1ed after convening 10 a pro- convention changed. jec1ed sys1em. described in chap1er 3. Asecoud angle of rotation. measured There are two primary conventions along north-south planes that intersect the used for specifying latitude and loogimde poles. is used to define a la1i1ude. Lalitudes (Figure 2-8). The first uses a leading letter. are specified as zero at the Equator. u,e Line N. S, E, or IV, 10 indica1e direction, fof. encircling 1he Ea11h that is equidistan1 from lowed by a number 10 indicate location. the North and Sou1h Poles. By convention, Northern latitudes are preceded by an N latitudes increase 10 maximum values of 90 and southern latitudes by an S, for exam- degrees in the 11011h and south. or, if signed. ple. N90°. SI0°. Lougimde values are pre-  Chapter 2: Data Models 29 Roto-t1on axis at the geogr aphic Nor th Pote. ). ?, 4' 90 Porollels. Eost-West lines of equol Geogroplic IOll!ude (,j,) porlqln: )..0. 4'.o U- 1 fi&"rt- l-6: Cc:m,·eu1iom wben n:fc1Ting to geographic Ultitudes Md lo11gitudC$. Mcridian!i arc lines nllUting nortb-soutb tba1 ba,·c coustant looajrud«. ParalMs arc Imes: n.wning cast·WC61 lb:u ban: ~.Stant latitudes. Latirudc i:s ZC':f"O Olt 1bc Equator. Loniirude is zero on tbc Ottcnwicb Mcridi.,11111,d undci1.ncd at 1.hc poles. b,cc.,usc all longitlld-iual mcridi!l1'5 iJ1tc111a:1 1licn: (i. ? in tbc figure). F1;urt l -7: Gcoat9phic coordi.na.t« o" a spbcrie11I (left) aild Car1csian (ri,ht) rcprCklltati.on. Notice that..cir- cles'' dcfotcd by ll 5 d~c nridiwi do 1.ot forut circles on the E:uth 's SW'f:ncc nc11t 1hc polc-s, 11.t shown on tht- sphcric.il representation (left figwc), but :ippca.r a, ci.rdc$ iu the dislorted Cn.ttcs:ian pie.I of gcogr,ip!i..ic coordi,. 11atcs (riaht), This figure illu!itratcs both that a) tbc sur!aoce disua:)Ce for a tmit of 1011.gitude changes depending on your location on Earth, a.iKi b) 11 Cat1c.s-Uln plot o( $C03N1phic coordinates is biahly dist01tcd. 30 GIS Fundamentals ceded by an E or W, for example WitO". Northern la1in1des are positive and southern Longin1des range from Oto 180 degrees latin1des are negative. and eastern longitudes east or west. Note 1ba1 the east and west positive and western longimdes negative. longitudes meet at 180 degrees. so that Latitudes vary from -90 degrees to 90 E 180° equals W 180°. degrees. and longitudes vary from - ISO Signed coordinates are 1he second com- degrees to 180 degrees. By this com ention. mon way to specify latitude and longitude. the longitudes "meel'' at the maximum and minimum values, so -180° equals t 80". Spheric.al geographic coordinates are North Pole often recorded in a degrees.minutes.seconds Nil0 , £84 (OMS) notation: N43° 35' 20" for 43 --"P-~·11· OC:s.,. degrees. 35 minutes. and 20 seconds of lati- tude. In OMS, each degree is made up of 60 minutes of arc. and e.acb minute is in mm divided into 60 seconds of arc (Figure 2-9). This yields 60 times 60. or 3600 seconds for each degree oflatitude or longirude. Spheri- cal coordinates may also be expressed as decimal degrees (DD). When usi ng DD. the degrees take the usual - ISO to ISO (longi- n,de) aud -90 to 90 (latinide) rru1ges. but minutes and seconds are reported as a deci- S62", E35 mal portion of a degree (from O to ·62' or.35.., 0.99999...). SOIJ1h Pote Con1·ersion be!IV~il DMS aud DD is Fleurt 2-8: SpheriC;J1J coord.i1utc~ of Luimdc :lnd shown in Figure 2-10. longitude 11.-re IUOtil often C: