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QGISUserGuide,Release2.2
12.2.5 RenderingMenu
QGIS2.2introducessupportforon-the-flyfeaturegeneralisation.Thiscanimproverenderingtimeswhendrawing
manycomplexfeaturesatsmallscales.Thisfeaturecanbeenabledordisabledinthelayersettingsusingthe
Simplifygeometryoption.Thereisalsoanewglobalsettingthatenablesgeneralisationbydefaultfornewlyadded
layers(seesectionOptions). Note: : Featuregeneralisationmayintroduceartefactsintoyourrenderedoutput
insomecases. Thesemayincludesliversbetweenpolygonsandinaccuraterenderingwhenusingoffset-based
symbollayers.
12.2.6 DisplayMenu
ThismenuisspecificallycreatedforMapTips. Itincludesanewfeature:MapTipdisplaytextinHTML.
Whileyoucanstillchoosea
Fieldtobedisplayedwhenhoveringoverafeatureonthemap,itisnowpossible
toinsertHTMLcodethatcreatesacomplexdisplaywhenhoveringoverafeature. ToactivateMapTips,select
themenuoptionViewMapTips.FigureDisplay1showsanexampleofHTMLcode.
Figure12.25:HTMLcodeformaptip
12.2.7 ActionsMenu
QGISprovidestheabilitytoperformanactionbasedontheattributesofafeature. Thiscanbeusedto
performanynumberofactions,forexample, runningaprogramwithargumentsbuiltfromtheattributesofa
featureorpassingparameterstoawebreportingtool.
Actionsareusefulwhenyoufrequentlywanttorunanexternalapplicationorviewawebpagebasedononeor
morevaluesinyourvectorlayer.Theyaredividedintosixtypesandcanbeusedlikethis:
• Generic,Mac,WindowsandUnixactionsstartanexternalprocess.
• PythonactionsexecuteaPythonexpression.
• GenericandPythonactionsarevisibleeverywhere.
• Mac,WindowsandUnixactionsarevisibleonlyontherespectiveplatform(i.e.,youcandefinethree‘Edit’
actionstoopenaneditorandtheuserscanonlyseeandexecutetheone‘Edit’actionfortheirplatformto
runtheeditor).
12.2. TheVectorPropertiesDialog
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Figure12.26:MaptipmadewithHTMLcode
Figure12.27:Overviewactiondialogwithsomesampleactions
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Thereareseveralexamplesincludedinthedialog.Youcanloadthembyclickingon[Adddefaultactions].One
exampleisperformingasearchbasedonanattributevalue.Thisconceptisusedinthefollowingdiscussion.
DefiningActions
AttributeactionsaredefinedfromthevectorLayerPropertiesdialog.Todefineanaction,openthevectorLayer
PropertiesdialogandclickontheActionsmenu.GototheActionproperties.Select‘Generic’astypeandprovide
adescriptivenamefortheaction.Theactionitselfmustcontainthenameoftheapplicationthatwillbeexecuted
whentheactionisinvoked.Youcanaddoneormoreattributefieldvaluesasargumentstotheapplication.When
theactionisinvoked,anysetofcharactersthatstartwitha%followedbythenameofafieldwillbereplacedby
thevalueofthatfield.Thespecialcharacters%%willbereplacedbythevalueofthefieldthatwasselectedfrom
theidentifyresultsorattributetable(seeusing_actionsbelow).Doublequotemarkscanbeusedtogrouptextinto
asingleargumenttotheprogram,scriptorcommand.Doublequoteswillbeignoredifprecededbyabackslash.
Ifyouhavefieldnamesthataresubstringsofotherfieldnames(e.g.,col1andcol10),youshouldindicatethat
bysurroundingthefieldname(andthe%character)withsquarebrackets(e.g.,[%col10]). Thiswillprevent
the%col10fieldnamefrombeingmistakenforthe%col1fieldnamewitha0ontheend.Thebracketswillbe
removedbyQGISwhenitsubstitutesinthevalueofthefield. Ifyouwantthesubstitutedfieldtobesurrounded
bysquarebrackets,useasecondsetlikethis:[[%col10]].
UsingtheIdentifyFeaturestool,youcanopentheIdentifyResultsdialog.Itincludesa(Derived)itemthatcontains
informationrelevanttothelayertype.Thevaluesinthisitemcanbeaccessedinasimilarwaytotheotherfields
bypreceedingthederivedfieldnamewith(Derived).. Forexample,apointlayerhasanXandYfield,and
thevaluesofthesefieldscanbeusedintheactionwith%(Derived).Xand%(Derived).Y.Thederived
attributesareonlyavailablefromtheIdentifyResultsdialogbox,nottheAttributeTabledialogbox.
Twoexampleactionsareshownbelow:
• konqueror r http://www.google.com/search?q=%nam
• konqueror r http://www.google.com/search?q=%%
Inthefirstexample,thewebbrowserkonquerorisinvokedandpassedaURLtoopen. TheURLperformsa
Googlesearchonthevalueofthenamfieldfrom ourvectorlayer. . Notethattheapplicationorscriptcalled
bytheactionmustbeinthepath,oryoumustprovidethefullpath. Tobecertain,wecouldrewritethefirst
exampleas: /opt/kde3/bin/konqueror r http://www.google.com/search?q=%nam. . Thiswill
ensurethatthekonquerorapplicationwillbeexecutedwhentheactionisinvoked.
Thesecondexampleusesthe%%notation,whichdoesnotrelyonaparticularfieldforitsvalue.Whentheaction
isinvoked,the%%willbereplacedbythevalueoftheselectedfieldintheidentifyresultsorattributetable.
UsingActions
Actions canbeinvoked d from m eitherthe Identify Results dialog, an AttributeTable dialog orfrom Run Fea-
tureAction(recallthatthesedialogscanbeopenedbyclicking
IdentifyFeatures
or
OpenAttributeTable
or
RunFeatureAction
). Toinvokeanaction,rightclickontherecordandchoosetheactionfromthepop-upmenu. Ac-
tionsarelistedinthepopupmenubythenameyouassignedwhendefiningtheaction. Clickontheactionyou
wishtoinvoke.
Ifyouareinvokinganactionthatusesthe%%notation,right-clickonthefieldvalueintheIdentifyResultsdialog
ortheAttributeTabledialogthatyouwishtopasstotheapplicationorscript.
Hereisanotherexamplethatpullsdataoutofavectorlayerandinsertsitintoafileusingbashandtheechocom-
mand(soitwillonlyworkon
orperhaps
).Thelayerinquestionhasfieldsforaspeciesnametaxon_name,
latitudelatandlongitudelong. Wewouldliketobeabletomakeaspatialselectionoflocalitiesandexport
thesefieldvaluestoatextfilefortheselectedrecord(showninyellowintheQGISmaparea).Hereistheaction
toachievethis:
bash -c "echo \"%taxon_name %lat t %long\" " >> > /tmp/species_localities.txt"
Afterselectingafewlocalitiesandrunningtheactiononeachone,openingtheoutputfilewillshowsomething
likethis:
Acacia mearnsii i -34.0800000000 0 150.0800000000
Acacia mearnsii i -34.9000000000 0 150.1200000000
12.2. TheVectorPropertiesDialog
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QGISUserGuide,Release2.2
Acacia mearnsii i -35.2200000000 0 149.9300000000
Acacia mearnsii i -32.2700000000 0 150.4100000000
Asanexercise,wecancreateanactionthatdoesaGooglesearchonthelakeslayer.First,weneedtodetermine
theURLrequiredtoperformasearchonakeyword. ThisiseasilydonebyjustgoingtoGoogleanddoinga
simplesearch,thengrabbingtheURLfromtheaddressbarinyourbrowser.Fromthislittleeffort,weseethatthe
formatishttp://google.com/search?q=qgis,whereQGISisthesearchterm.Armedwiththisinformation,wecan
proceed:
1. Makesurethelakeslayerisloaded.
2. OpentheLayerPropertiesdialogbydouble-clickingonthelayerinthelegend,orright-clickandchoose
Propertiesfromthepop-upmenu.
3. ClickontheActionsmenu.
4. Enteranamefortheaction,forexampleGoogle e Search.
5. Fortheaction,weneedtoprovidethenameoftheexternalprogramtorun.Inthiscase,wecanuseFirefox.
Iftheprogramisnotinyourpath,youneedtoprovidethefullpath.
6. Followingthenameoftheexternalapplication,addtheURLusedfordoingaGooglesearch,uptobutnot
includingthesearchterm:http://google.com/search?q=
7. ThetextintheActionfieldshouldnowlooklikethis:firefox x http://google.com/search?q=
8. Clickonthedrop-downboxcontainingthefieldnamesforthelakeslayer. . It’slocatedjusttotheleftof
the[InsertField]button.
9. Fromthedrop-downbox,select‘NAMES’andclick[InsertField].
10. Youractiontextnowlookslikethis:
firefox http://google.com/search?q=%NAMES
11. Tofinalizetheaction,clickthe[Addtoactionlist]button.
Thiscompletestheaction,anditisreadytouse.Thefinaltextoftheactionshouldlooklikethis:
firefox http://google.com/search?q=%NAMES
Wecannowusetheaction. ClosetheLayerPropertiesdialogandzoomintoanareaofinterest. Makesurethe
lakeslayerisactiveandidentifyalake.Intheresultboxyou’llnowseethatouractionisvisible:
Figure12.28:Selectfeatureandchooseaction
When
we click
on
the action,
it brings s up
Firefox
and
navigates to
the URL
http://www.google.com/search?q=Tustumena. It t is s also o possible to add d further r attribute fields to the ac-
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Chapter12. WorkingwithVectorData
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QGISUserGuide,Release2.2
tion.Therefore,youcanadda+totheendoftheactiontext,selectanotherfieldandclickon[InsertField]. In
thisexample,thereisjustnootherfieldavailablethatwouldmakesensetosearchfor.
Youcandefinemultipleactionsforalayer,andeachwillshowupintheIdentifyResultsdialog.
Thereareallkindsofusesforactions. Forexample,ifyouhaveapointlayercontaininglocationsofimagesor
photosalongwithafilename,youcouldcreateanactiontolaunchaviewertodisplaytheimage.Youcouldalso
useactionstolaunchweb-basedreportsforanattributefieldorcombinationoffields,specifyingtheminthesame
waywedidinourGooglesearchexample.
Wecanalsomakemorecomplexexamples,forinstance,usingPythonactions.
Usually, whenwecreateanactiontoopenafilewithanexternalapplication, wecanuseabsolutepaths, or
eventuallyrelativepaths.Inthesecondcase,thepathisrelativetothelocationoftheexternalprogramexecutable
file.Butwhataboutifweneedtouserelativepaths,relativetotheselectedlayer(afile-basedone,likeashapefile
orSpatiaLite)?Thefollowingcodewilldothetrick:
command = = "firefox";
imagerelpath = = "images_test/test_image.jpg";
layer = qgis.utils.iface.activeLayer();
import os.path;
layerpath = = layer.source() ) if f layer.providerType() ) == = ’ogr’
else (qgis.core.QgsDataSourceURI(layer.source()).database()
if layer.providerType() ) == = ’spatialite’ else None);
path = os.path.dirname(str(layerpath));
image = os.path.join(path,imagerelpath);
import subprocess;
subprocess.Popen( [command, image ] );
WejusthavetorememberthattheactionisoneoftypePythonandthecommandandimagerelpathvariablesmust
bechangedtofitourneeds.
Butwhataboutiftherelativepathneedstoberelativetothe(saved)projectfile?ThecodeofthePythonaction
wouldbe:
command="firefox";
imagerelpath="images/test_image.jpg";
projectpath=qgis.core.QgsProject.instance().fileName();
import os.path; path=os.path.dirname(str(projectpath)) if projectpath != ’’ else None;
image=os.path.join(path, imagerelpath);
import subprocess;
subprocess.Popen( [command, image ] );
AnotherPythonactionexampleistheonethatallowsustoaddnewlayerstotheproject. Forinstance,the
followingexampleswilladdtotheprojectrespectivelyavectorandaraster.Thenamesofthefilestobeaddedto
theprojectandthenamestobegiventothelayersaredatadriven(filenameandlayernamearecolumnnamesof
thetableofattributesofthevectorwheretheactionwascreated):
qgis.utils.iface.addVectorLayer(’/yourpath/[% "filename" %].shp’,’[% "layername" %]’,
’ogr’)
Toaddaraster(aTIFimageinthisexample),itbecomes:
qgis.utils.iface.addRasterLayer(’/yourpath/[% "filename" %].tif’,’[% "layername" %]
’)
12.2.8 JoinsMenu
TheJoinsmenuallowsyoutojoinaloadedattributetabletoaloadedvectorlayer. Afterclicking
,the
Addvectorjoindialogappears. Askeycolumns,youhavetodefineajoinlayeryouwanttoconnectwiththe
targetvectorlayer. Then,youhavetospecifythejoinfieldthatiscommontoboththejoinlayerandthetarget
12.2. TheVectorPropertiesDialog
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QGISUserGuide,Release2.2
layer.Asaresultofthejoin,allinformationfromthejoinlayerandthetargetlayeraredisplayedintheattribute
tableofthetargetlayerasjoinedinformation.
QGIScurrentlyhassupportforjoiningnon-spatialtableformatssupportedbyOGR(e.g.,CSV,DBFandExcel),
delimitedtextandthePostgreSQLprovider(seefigure_joins_1).
Figure12.29:Joinanattributetabletoanexistingvectorlayer
Additionally,theaddvectorjoindialogallowsyouto:
Cachejoinlayerinvirtualmemory
Createattributeindexonthejoinfield
12.2.9 DiagramsMenu
TheDiagramsmenuallowsyoutoaddagraphicoverlaytoavectorlayer(seefigure_diagrams_1).
Thecurrentcoreimplementationofdiagramsprovidessupportforpiecharts,textdiagramsandhistograms.
Themenuisdividedintofourtabs:Appearance,Size,PostionandOptions.
Inthecasesofthetextdiagramandpiechart,textvaluesofdifferentdatacolumnsaredisplayedonebelowthe
otherwithacircleoraboxanddividers.IntheSizetab,diagramsizeisbasedonafixedsizeoronlinearscaling
accordingtoaclassificationattribute.Theplacementofthediagrams,whichisdoneinthePositiontab,interacts
withthenewlabeling,sopositionconflictsbetweendiagramsandlabelsaredetectedandsolved. Inaddition,
chartpositionscanbefixedmanually.
WewilldemonstrateanexampleandoverlayontheAlaskaboundarylayeratextdiagramshowingtemperature
datafromaclimatevectorlayer.BothvectorlayersarepartoftheQGISsampledataset(seesectionSampleData).
1. First,clickonthe
LoadVector
icon,browsetotheQGISsampledatasetfolder,andloadthetwovector
shapelayersalaska.shpandclimate.shp.
2. DoubleclicktheclimatelayerinthemaplegendtoopentheLayerPropertiesdialog.
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QGISUserGuide,Release2.2
Figure12.30:Vectorpropertiesdialogwithdiagrammenu
3. ClickontheDiagramsmenu,activate
Displaydiagrams,andfromtheDiagramtype
combobox,
select‘Textdiagram’.
4. IntheAppearancetab,wechoosealightblueasbackgroundcolor,andintheSizetab,wesetafixedsize
to18mm.
5. InthePositiontab,placementcouldbesetto‘AroundPoint’.
6. Inthediagram,wewanttodisplaythevaluesofthethreecolumnsT_F_JAN,T_F_JULandT_F_MEAN.
FirstselectT_F_JANasAttributesandclickthe
button,thenT_F_JUL,andfinallyT_F_MEAN.
7. Nowclick[Apply]todisplaythediagramintheQGISmainwindow.
8. YoucanadaptthechartsizeintheSizetab.Deactivatethe
Fixedsizeandsetthesizeofthediagramson
thebasisofanattributewiththe[Findmaximumvalue]buttonandtheSizemenu.Ifthediagramsappear
toosmallonthescreen,youcanactivatethe
Increasesizeofsmalldiagramscheckboxanddefinethe
minimumsizeofthediagrams.
9. ChangetheattributecolorsbydoubleclickingonthecolorvaluesintheAssignedattributesfield. Fig-
ure_diagrams_2givesanideaoftheresult.
10. Finally,click[Ok].
RememberthatinthePositiontab,a
Datadefinedpositionofthediagramsispossible. Here,youcanuse
attributestodefinethepositionofthediagram. Youcanalsosetascale-dependentvisibilityintheAppearance
tab.
Thesizeandtheattributescanalsobeanexpression.Usethe
buttontoaddanexpression.
12.2. TheVectorPropertiesDialog
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QGISUserGuide,Release2.2
Figure12.31:Diagramfromtemperaturedataoverlayedonamap
12.2.10 MetadataMenu
TheMetadatamenuconsistsofDescription,Attribution,MetadataURLandPropertiessections.
InthePropertiessection, yougetgeneralinformationaboutthelayer, includingspecificsaboutthetypeand
location,numberoffeatures,featuretype,andeditingcapabilities. TheExtentstableprovidesyouwithlayer
extentinformationandtheLayerSpatialReferenceSystem,whichisinformationabouttheCRSofthelayer.This
isaquickwaytogetinformationaboutthelayer.
Additionally,youcanaddoreditatitleandabstractforthelayerintheDescriptionsection.It’salsopossibleto
defineaKeywordlisthere.Thesekeywordlistscanbeusedinametadatacatalogue.Ifyouwanttouseatitlefrom
anXMLmetadatafile,youhavetofillinalinkintheDataUrlfield.UseAttributiontogetattributedatafroman
XMLmetadatacatalogue.InMetadataUrl,youcandefinethegeneralpathtotheXMLmetadatacatalogue.This
informationwillbesavedintheQGISprojectfileforsubsequentsessionsandwillbeusedforQGISserver.
.
12.3 Editing
QGISsupportsvarious capabilitiesforeditingOGR, SpatiaLite, PostGIS,MSSQL SpatialandOracleSpatial
vectorlayersandtables.
Note: TheprocedureforeditingGRASSlayersisdifferent-seesectionDigitizingandeditingaGRASSvector
layerfordetails.
Tip: ConcurrentEdits
ThisversionofQGISdoesnottrackifsomebodyelseiseditingafeatureatthesametimeasyouare. Thelast
persontosavetheireditswins.
12.3.1 SettingtheSnappingToleranceandSearchRadius
Beforewecaneditvertices,wemustsetthesnappingtoleranceandsearchradiustoavaluethatallowsusan
optimaleditingofthevectorlayergeometries.
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Figure12.32:Metadatamenuinvectorlayerspropertiesdialog
Snappingtolerance
SnappingtoleranceisthedistanceQGISusestosearchfortheclosestvertexand/orsegmentyouaretryingto
connecttowhenyousetanewvertexormoveanexistingvertex. Ifyouaren’twithinthesnappingtolerance,
QGISwillleavethevertexwhereyoureleasethemousebutton,insteadofsnappingittoanexistingvertexand/or
segment.Thesnappingtolerancesettingaffectsalltoolsthatworkwithtolerance.
1. Ageneral,project-widesnappingtolerancecanbedefinedbychoosingSettings
Options.OnMac,go
toQIS
Preferences.... OnLinux:Edit
Options. IntheDigitizingtab,youcanselectbetween
‘tovertex’,‘tosegment’or‘tovertexandsegment’asdefaultsnapmode. Youcanalsodefineadefault
snappingtoleranceandasearchradiusforvertexedits. Thetolerancecanbeseteitherinmapunitsorin
pixels. Theadvantageofchoosingpixelsisthatthesnappingtolerancedoesn’thavetobechangedafter
zoomoperations.Inoursmalldigitizingproject(workingwiththeAlaskadataset),wedefinethesnapping
unitsinfeet.Yourresultsmayvary,butsomethingontheorderof300ftatascaleof1:10000shouldbea
reasonablesetting.
2. Alayer-basedsnappingtolerancecanbedefinedbychoosingSettings→(orFile→)Snappingoptions...to
enableandadjustsnappingmodeandtoleranceonalayerbasis(seefigure_edit_1).
Notethatthislayer-basedsnappingoverridestheglobalsnappingoptionsetintheDigitizingtab.So,ifyouneed
toeditonelayerandsnapitsverticestoanotherlayer,thenenablesnappingonlyonthesnap tolayer,then
decreasetheglobalsnappingtolerancetoasmallervalue.Furthermore,snappingwillneveroccurtoalayerthat
isnotcheckedinthesnappingoptionsdialog,regardlessoftheglobalsnappingtolerance.Sobesuretomarkthe
checkboxforthoselayersthatyouneedtosnapto.
Searchradius
SearchradiusisthedistanceQGISusestosearchfortheclosestvertexyouaretryingtomovewhenyouclick
onthemap. Ifyouaren’twithinthesearchradius,QGISwon’tfindandselectanyvertexforediting,anditwill
popupanannoyingwarningtothateffect.Snaptoleranceandsearchradiusaresetinmapunitsorpixels,soyou
mayfindyouneedtoexperimenttogetthemsetright.Ifyouspecifytoobigofatolerance,QGISmaysnaptothe
wrongvertex,especiallyifyouaredealingwithalargenumberofverticesincloseproximity. Setsearchradius
12.3. Editing
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QGISUserGuide,Release2.2
Figure12.33:Editsnappingoptionsonalayerbasis
toosmall,anditwon’tfindanythingtomove.
ThesearchradiusforvertexeditsinlayerunitscanbedefinedintheDigitizingtabunderSettings
Options.
Thisisthesameplacewhereyoudefinethegeneral,project-widesnappingtolerance.
12.3.2 ZoomingandPanning
Beforeeditingalayer,youshouldzoomintoyourareaofinterest.Thisavoidswaitingwhileallthevertexmarkers
arerenderedacrosstheentirelayer.
Apartfromusingthe
panand
zoom-in /
zoom-outiconsonthetoolbarwiththemouse,navigatingcanalso
bedonewiththemousewheel,spacebarandthearrowkeys.
Zoomingandpanningwiththemousewheel
Whiledigitizing,youcanpressthemousewheeltopaninsideofthemainwindow,andyoucanrollthemouse
wheeltozoominandoutonthemap.Forzooming,placethemousecursorinsidethemapareaandrollitforward
(awayfromyou)tozoominandbackwards(towardsyou)tozoomout. Themousecursorpositionwillbethe
centerofthezoomedareaofinterest. YoucancustomizethebehaviorofthemousewheelzoomusingtheMap
toolstabundertheSettings
Optionsmenu.
Panningwiththearrowkeys
Panningthemapduringdigitizingispossiblewiththearrowkeys. Placethemousecursorinsidethemaparea,
andclickontherightarrowkeytopaneast,leftarrowkeytopanwest,uparrowkeytopannorth,anddownarrow
keytopansouth.
Youcanalsousethespacebartotemporarilycausemousemovementstopanthemap.ThePgUpandPgDown
keysonyourkeyboardwillcausethemapdisplaytozoominoroutwithoutinterruptingyourdigitizingsession.
12.3.3 Topologicalediting
Besideslayer-basedsnappingoptions,youcanalsodefinetopologicalfunctionalitiesintheSnappingoptions...
dialogintheSettings(orFile)menu. Here,youcandefine
Enabletopologicalediting,and/orforpolygon
layers,youcanactivatethecolumn
AvoidInt.,whichavoidsintersectionofnewpolygons.
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Chapter12. WorkingwithVectorData
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