95

al Positioxing System as a tool for ecoslyslem studies at the ndscape level: an application in the Spanish Mediterranean

Ignacio Zavata Miguel A. Zavalab rmem o~ca~~~~r~p~~~. E~gaweerutg, E U I i Agricola, tfnlversrdad Polrrknrca de Madrid, Mcdrrd, Spam

bCfnrl o/Forest Pash&gy E T S hgenreros de Mantes, b’nwersrdad Polrrt?nrca de Madnd, Madrid, Spam

(Accepted 8 January 1993 )

Global Posttionrng System (GPS) BS a satelltte-based method whtch auous the cooromates of points to be determined wathtn centimeters or meters of accuracy. depending on the measunng techmoues used. GPS also allows boundartes to be

otted and areas to be measured m stgmficantly less ttme and m a much easlzr way than tradtttonal techniques. GPS can be used as a first-hand tw! f~i a better understanding of ecologrcal processes and human interferences, and for a more efkrent management and conservation effort of the ecosystem components. GPS is espectally sutted for those apphcattons involving landscape fragmentation and moszw pattern. The method used tn thts study IS Dtfferential GPS (DGPS). It Involves placing a GPS receiver at one or more known locatlons (the momtor stations) and computing corrections for the range errors observed m the satellite signals. In princtple, these correctrons are caicuiared by _dbtracrutg the observed pseudo-ranges from ranges computed wth the known station coordmates. Our appltcatton focuses on the problem of determining the infected areas of Arceurhobrum oxycedrr DC. M. Bteb (an Bberian dwarf mtstletoe found on Junipewr oxycedrw !_. ) within Parque Regonal del Alto Manzanares ( Madnd ), a protected ecological area. Thts park fxes the typical problems of Mediterranean landscapes next to brg metropolitan areas GPS allows the interactive study of dwarf mistletoe spreading and perturbation agents such as wtldfires and human Influence.

Introduction and objectives

The Cuenca de1 Alto Manzanares Regional Park is a protected area close to Madrid city. It faces the typical problems of other Mediterra- nean landscapes next to big metropolitan areas. In a rapidly changing environment like this, with all kinds of perturbations, managers need fast tools to handle alI this information.

Global Positioning System (GPS) is a fast and reliable method for the scale at which management operates. On the Rae hand it gives the chance of plotting boundaries precisely and speedily, improving hand-sketches and old aerial photographs. On the other hand, it of- fers many possibilities to scientists interested in spatial distributions of organisms. On both

*Corresponding author.

levels GPS interfacing system allows combi- nations with Geographic Information Systems (GIS) which prove very useful.

In this particular case GPS was used to study the local distribution of juniper dwarf mistle- toe (Arwuthobium oAycedri DC. M. Bieb). It is an aerial dicotyledonous parasite (Family Loranthaceae). In Spain it is found on Juni- perus oxycedrus L. and accidentally cn Thuja orientalis L. ( Beer, 195 1) and Cupressus ariz- onica E.L. Green (Rios Ins& 1984). Shoots average 5-10 cm long, but up to 20 cm; yellow green; verticillately branched ( Kuijt, 1970 ) ; basal shoots 1.5-2 mm in diameter, third in- ternode 5-9 m;n long; staminate flowers 1.5- 2.5 mm in diameter, pe%nth mostly iri-, but sometimes di- or tetramerous. Anthesis occurs in September and October and seed dispersal in falI (Heinricher , 1 S 15 ). Although it has been

0 1993 Elsevter Science Publtshers B.V. All nghts reserved 0169-2046/93/$06.00

cntensivcly studied in thz Pacific Northwest in the Ji_JS, little has bsen done in Spain. One of the reasons is that it does noi cause timber lossc~ in the Ibcrian Peninsula. $ince its hosts ;u*c not commercial species.

escribed as very local (Tu fl et al., 1964), .&tet~tMillrn oxycedri is the +t widely dis- tributeil dwarf nrist!etoe. Dw ~~iStktOt3 in

genera1 are considered anno+ ~1 I-Iowever its role in vegctal sue ology in non-commercial for0 is relatively t.nknown. It seems clear, howe\ iper woodlands are to be preser tletoe should be controlled.

For a first approach to the proMem GPS is a good candidate. It could be a usei‘ul tool for se- lecting areas and plotting !ocal distribution. The objectives are to test GPS at the next levels.

Use GPS for plotting boundaries of affected areas. roads and wildfire edges, dividing the area of study into parts according to host vigor.

Study accuracv compared with classical top- - ographic methods.

Try plot relocation in the fit L. taking into account selective availahilit> :Ind other dra:jbachs.

Incorporate GPS files into 7 “S fix agement purposes.

Use GPS to plot host distributio (‘heck otit relocation on a tree )1\ tree basis. St+ the suitability of this kind of applica-

tion taking into account the time spent in field and office work.

aterids and methods

Three different areas have beer! _kosen, two of them inside the Park and one oL:.side it, but

in its surroundings, taking into account t

vigor of th * junipers and their grade of affec- tion by the &.x-f mistletoe.

In the selection the usage and activities that take place in the area, governed b;? the 1 management framework of the Region have also been taken into account.

1M I I

Station In the middle of the park. inside one of the B, areas, called ‘producing local farming park’ with a high capacity production. It has a high ecological value and a poor nature conser- vation state. Producing farming activities xe allowed in this B2 area.

Jldnipers. Affected trees with vigor below av- erage. heavily infected by the parasite. Qt~~t~s i!ex L. and Cistzls hdanifer L. are dominating over the junipers.

Orhc~ cYzaracleris/xJ. It is an area that limrts with a recent fire ( 1990) and a military usage area. It is crossed by paths with frequent transit of drive-land vehicles at the weekends. It is a cattle-grazed area.

-ME-l II

Sztuatiorz. Northeast of the park inside an area B, called ‘Protecting local farming park’ of high ecological value and high nature conservation and qualities of its ecosystems.

Junipm. Trees of average vigor are affected

warf mistletoe. Jun- ers are the dominant species without woody

It is an area that be- a mount-free provision of t!:r’ county

cry close to it l 11 is often r day-tr;ppcrs Z+c ~VY,-Cl~ends. area. It is an area of transit

ominiums at the

Jrrniplers. Trees of above average vigor in gen- cral very little affected by dwarf mistletoe. Slight shrub understory.

Other chwcteristics. It is a horse-grazed area. Private property.

Severity of Arceuthobium infection was re- corded using 2 method based on the dwarf mistletoe rating system (Hawksworth, 1977). Each tree was divided vertically into two parts that were r&ed 0, 1, or 2 (0, dwarf mistletoe absent; 1, present; 2, present in at least half the branches). The total of the ratings for the crown halves was the tree rating (between 0 and 4).

Usage qf GPS

GPS has been used to survey the areas and situate the trees inside them, as well as to sur- vey the roads, fire boundaries and cultivated land. In all the observations the reference re- ceiver used has been an eight-channel Trimble 4OOOST receiver of Ll frequency. A six-chan-

, nel Trimble Pathfinder receiver, and C/A code has always been mote receiver.

D&‘inwtlal GPS

GPS provides an accuracy in

L 1 frequency used as a re-

the position, using only one receiver with a C/A code of 25 m 2dRMS, if the selective availability is un- workable. But nowadays we: have to assume that the US Department of Defense (DOD) is causing a deliberate &gradation in selective availability (having an effect directly on the clocks of the satellites or manipulation of the ephemeris data). This causes the satellite pseudo-ranges to vary quickly (period of mm- utes ) with limited amplitl:& and a very large but slowly varying bias in trl - position solution (Kremer et al., 1990). The expected accuracy in the position is about 100 m 2dRMS, or even worse as has been found in some cases. But be- sides this important error source, there are others that also affect our position (Table 1).

It is, therefore, quite clear that we will not be able to use only one receiver in a stand-alone way for our goals and that we will have to use the Differential GPS (DGPS) technique, which improves the accuracy. This method 5as started from the assumption of some kinds of errors that degrade the working and accuracy of the system which are familiar to al users if

Table I Differential errors under selectwe avallablllty ( Ackroyd and Lonmer, 1990 j

Error source Stand-alone Dlfferentlal

Space segment Clock mstabrhty 15.0 m Om

Ephemerrs errors 40 0 m Om

Orbit errors 5.0 m Om User segment Ionospheric delay 13.0 m 1.0 m

Troposphenc delay 3.0 m 0.5 m

Multi-path 2.0 m 2.0 m

Receiver noise 2.0 m 2.8 m

Total root sum squared 44.8 I-K. 3.6

Pig. I. Compaflson of ranges.

I -

J3g. 2. DGPS concept.

these errors are not too separated. If these er- rors can be calculated at one point, one can de- termine some corrections and the latter can be applied to the rest. In this way one eliminates or reduces the errors. This implies that one of the points must be of known coordinates and that one is watching the same four satellites at the same time from this point and the un- known one. So one situates a receiver (refer- ence receiver) on a point of known coordi- nates, usually a survey monument or control point in the same reference system that GPS operates, that is to say WGS84, and also in the system one would like to transform the calcu-

Table 2 Coordmatcs of reference recwcr

Iat 40’26’34” 1 SSS N Ion 3*43’47” 3575 W Height 707 112 m

Y4S51196 57 1’ - 3 I 6248.4s 24115989 23

&DO Jat 40’26’38”48 N Jon 3”43’42”68 W J-hght 633.92 m

X485 1282.27 Y -316143.43 Z4116111.S7

X4382 15.239 Y 447 1373.43s

c-_-__ _ .- I

Fig. 3. Posrtmns eclery second of he file EZS (radm=45.5

W.

Table 3

Pssltmns rejected

Pcxrtions Rejected

.-izEA I

Day 036 78 IO 12.8 Day 049 60 0 0

.1RE4 II

Day 058 28 6 21.4 Day 076 63 3 4.8 Day 08 I 98 5 5.1 Day 098 79 6 7.6

lated coordinates for each point (ED50 in our case) and one or more receivers (remote re- ceivers) on the points to survey. The system will work while one is watching simultane- ously from the reference and the remote receivers.

The reference receiver is calculating its po- sition using the signal of the satellites as well as the remote receiver but the position of the for- mer can be compared in every single instant (each 1, 5, 10, or more seconds) with the known one. Therefore one can determine the error in the calculation. One can then calculate

99

the corrections and apply them to the calcu- lated positions by the remote receivers. With this differential method the errors can be elim- inated or reduced owing to the satellite clock instability, the ephemeris errors, orbit errors and the ones produced by the ionospheric de- lay, tropospheric delay and &he errors pro- duced by the receiver itself (Table 1). The er- rors produced by the multi-path effect c;rnuot, be corrected with a differential technique, so the observations must be avoided as much as possible in places where the waves can suffer this type of disturbance.

There are two ways to 4efine and apply the corrections in DGPS. The easiest one is using the position corrections. A reference receiver at a known point calculates a position from at least four satellites. This position is compared with the known position and the corrections are computed as delta X, Y and 2.

Another more complicated method consists cf determining the corrections in the calcu- lated pseudo-ranges from the reference re- ceiver to the different satellites and applying them to the calculated pseudo-ranges from the remote receiver (calculation of delta pseudo- ranges). This is obtained by comparing the true range existing between the receiver and the different satellites, knowing the Cartesian po- sition (X, Y, 2) of the reference receiver and the cartesian position Of the sateKites in the broadcast message with the pseudo-range mea- sured using the C/A code (Fig. 1). This is the method w ha*:e used in the determination of the coordizatcs sf the junipers. This method has the advantage that the remote receiver is totally independent in its selection of satellites to compute its position, since the reference re- ceiver is supposed to be calculating the COITX-

tions for all satellites in view ( Fig. 2 ) - To sum up, with the DGPS you get an accu-

racy of about 2-5 m 2dRMS, enough for this survey to cope with, and the receivers between them can be far from each other at several hundred kilometers.

El E6 E 10 E 26 ES2 E40 E 60

E Numb3

80

40

’ -.

y -1 1 1 .Y . ‘I, ’ I \

SrciDev (ml

.l-=-----

El E6 E 1C E26 E32 E40 E 6iI

E Nunmr

StdDev (ml -_ -1

I 30.846 I

t 24.?29

nL

X

FIN 5. Standard dw r,wolr of reference statlon ( Da> OS 1 ).

In every survey in which one uses GPS, the preparation is a very important phase and has an important bearing on the final results. This planning includes a detailed survey of the con- stellation, determining the moments when the number of satellites and their geometry (PDOP) are most favorable over the surveyed area.

The 4000ST receiver was situated on the ref- erence station built in the E.U. Ingenieria Tec-

Y

nica Agricola at the University of Madrid, whose coordinates were calculated through static relative positioning and conventional survey in WGS84 and ED50 systems (Table

2). To ensure the removal of the effects of the

selective availability the interval of record po- sition was fixed as 10 s in the reference re- ceiver. The remote receiver recorded positions every second. The cbservation time in each tree was established as 30 s. The antenna was situ-

AREA I

101

,3 UNINFECTED

. VFECTED

ated on a range pole long enough to rise over the top of the tree, so the branches would not affect the reception of the signal. This was not always possible owing to the great height of some trees, so the antenna was situated among

es getting a good reception in these cases too (all the observations were carried out with a high number of satellites and the six- channel receiver was very reliable in these cases).

While the GPS observation was being made, the condition of the tree was determined and it was marked with a small signal to be recog- nized later. In this way observations were not repeated. Height and the diameter at breast height were a!50 measured in order to use these data in future surveys of dwarf mistletoe. Every tree was assigned to a different file in the re- mote receiver but without turning this off be- tween measurements. This would avoid the undesirable errors produced while warming up. Nevertheless, it seems that it was not managed completely, since some files, before being cor- rected, showed their calculated positions

(about 30 in each tree) to be wrong (Fig. 3 ). For the survey of the roads and borders of the different areas included in the study, thz inter- val for the positioning of the remote receiver was established as the distance walked on foot in 3 s.

Da f a processil ‘g All the data processing of the observation up

to the final graphics can be arranged in the fol- lowing steps.

( 1) To dump the data from the receivers to the computer.

(2 ) Calculation and applying of the differ- ential corrections.

(3) To erase the files with uncorrected positions.

(4) To get the average position from each file.

(5) To sum the files in each of the three sur- veyed areas.

( 6 ) Final plotting.

In the process of dumping the data the stan- dard deviations were obtained from the calcu-

lated position in the reference receiver as well ote one before the correction (Ta-

The calculation and applying of the differ- corrections were done the utility of nder utility of the C’R EC Plus pro- Trimble Navigation, 199 1). Some of the

es could not be corrected with the reference receiver. Nevertheless, these files were elimi- nated having no graphic representation left.

he Pathfinder program (Trimble Naviga- tion, 199 1) was used to determine the average position of every corrected file and to sum up all the files corresponding 10 the junipers of every area. Therefore only a corrected position of every juniper was obtained.

ILater, diflerent symbols and ZO~OE were ZS- signed to the different degree of affection of each tree and definitive maps of every area surveyed were drawn.

11 plot of land of about 100 m x 100 m was measured in the second surveyed area where 12 points were situated. These were surveyed with a total station Nikon DTMA 10 Lg and DGPS. The range pole with the antenna of the remote receiver was sited at the same point where the previous range pole was placed with the comer-cube retroreflector. The observa- tion time in each point was 30 s us well as in the positioning of the junipers.

Before applying the differential corrections, standard deviations of the obtained positions in every point was detemined. This let us know in which points the effects of the selective availability was tiecting in an obvious way the calculation of its positions (Fig. 4 ). So in some points, separated only by minutes (the condi- tions did not change in a substantial way) very different standard deviations were obtained for the calculated positions for about the 30 s in

which the observation lasted in each one. In t same way, the standard deviation to the cal- culated positions in the reference receiver were determined ( Fig. 5 ).

After applying the dimerential these errors were reduced, even t points could be corrected (Ta corrected points were used for the final maps of the three

conventinn?! pn+- _ _..od after the pro GPS observations and the conventis the resul:s obtained were co checked so errors with DGPS were as have been expected (Fig. 9)) eve test was done with an excellent PDisP -with six satellites over the horizon during the time of the test. It was satisfactory taking into account that it was only expected to check the relative position among points.

AREA /j

0 2OC m

0 WtiFtlcTED

$ INFECTED

DEAD

Fig. 7. Area II. Host and parasite distribution.

AREA III

0 IJNINFECTED

Fig. 8. Area III. Has? and parasite distribution.

Fig, 9. DGPS and Total Station comparison.

The DGPS method has turned out to be sat- isfactory for the type of surveys in which a cen- timetric accuracy is not needed. It is a remark- able improvement on works done only with a sketch map. Although the field work is really easy, the bureau work, at the moment. needs much more time. Anyway it is quite clear that good previous planning before doing the ?vork. saves much time in the data postprocessing. However we will always be subjected to the disadvantages of a developing system and property of the DoD. In spite of this, the DGPS lets us tackle works which would not be possi- ble with conventional surveying methods, as in the cases of working quickly in rough areas with dense vegetation as is the case of our surveyed areas.

On the other hand, the DGPS has turned out to be useful as a method of giving coordinates to isolated points contrary to the usual usage for shipping, always taking into account the accuracy limits. Concerning relocation it tuned out to be possible for plots, but under the current selective availability conditions tree relocation is not worthwhile unless using DGPS in real time.

Acknow!edgments

The authors wish to thank Enrique Perez and Angel Herranz for their support during the field work, and to Cristina Martinez for her help in the preparation of the manuscript.

efcrences

Ackroyd. N. and Lorimcr. R.. 1990. Global N:.lvigation. A GPS User’s Guide. LLoyd’s of London Press. London.

Beer. A.. 195 1. Die Zwtrg-oder Wacholder Mistel (.drc~rr- tltc&rrt~?r ) under ihre kiinstlichc Aufzucht. Garten-Z: Flora. 74( 2): 13-14.

Hawksworth. F.G.. 1977. The &class dwarf mistletoe rating system. US Dep. Agric. For. Serv. Gen. Tech. Rep. RM- 48. 7 pp.

Heinricher. E.. 19 15. Uber Bau und Biologie der Bluten volr ..irc.cui~~c)l,rltt?l c~_v_r~wiri ( DC. ) M.B. Akad. Wiss. Wien. Math.-natur.kl.sitzungsber. Abt. I. 124: 48 l-504.

Krcmer. G.T.. Kalafus. R.M.. Loomis. P.V. and Reynolds, J-C., 1990. The effect of selective availability on diffcren- tial GPS corrections. NAVIGATION. J. Inst. Navig.. 37 (No. 1, Spring): 39-52.

Kuijt. Job., 1970. A systematic study ofbranching patterns in dwarf mistletoe (.lr.~.c’lctllr,l,jiil)z ). Torrel Bot. Club Mcm., 22(A): l-38.

Rios-Ir?stia, M.V., 1984. Aggressiveness of .-lrcctrthohium ox- yaw-i in Spain. The Golden Bough (Royal Botanic Gar- dens, Kew, UK ) no. 5: 4.

Trimble Navigation Ltd., 199 I. GPS Survey Software. Re- lease notes. TRIMVEC-PLUS, Rev. Dl. Sunnyvale, CA.

Tutin et al. 1964. Flora Europaea, Cambridge University Press. Cambridge, Vol. I. 464 pp.