Developing a digital soil mapping protocol for southern Africa using case studies

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Date
2013-06
Authors
Van Zijl, G. M.
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University of the Free State
Abstract
English: Although there is an increasing need for spatial soil information, traditional methods of soil survey are too cumbersome and expensive to supply in that need. Digital soil mapping (DSM) methods can fulfil that need. Internationally, DSM is moving from the research to the production phase. As soil-landscape interaction and availability of data varies between locations, local DSM research is needed to make its application practical. This research aims to produce a working DSM protocol which can be used for mapping large areas of land in southern Africa. The protocol must meet soil surveyors where they are at, being easy enough to follow, while also allowing for the creation of products needed by industry. To keep the link with industry’s needs, a case study approach was followed. Four case studies were done in succession, with the protocol being improved with every case study. The case studies cover an array of challenges faced by soil surveyors. In the first case study a baseline protocol was created when two land types near Madadeni were disaggregated in a series of soil maps. With each map, more information was incorporated when creating the map. For Map 1 only the land type inventory and terrain analysis were used. A reconnaissance field visit with the land type surveyor was added for the second map. Field work and a simplified soil association legend proved to improve the map accuracy for Maps 3 and 4, which were created using 30% and 60% of the observations points as training data respectively. The accuracy of the maps increased when more information was utilized. Map 1 reached an accuracy of 35%, while Map 4 achieved a commendable accuracy of 67%. Principles which emerged was that field work is critical to DSM, more data input improves the output and that simplifying the map legend improves the accuracy of the map. An unrealistic demand for a soil survey of 37 000 ha of land in the Tete Province, northern Mozambique, possibly infested with land mines, in 8 working days by two persons, created an opportunity to apply the soil-land inference model (SoLIM) as a digital soil mapping tool. Dividing the area into smaller areas where unique soil distribution rules would apply (homogeneous areas, HA’s) was introduced. A free survey was conducted along the available roads of the area. The final soil map for 15 000 ha had an accuracy of 69%. A principle which emerged was that inaccessible areas can be mapped, provided that they occur within surveyed HA’s. Near Namarroi, Mozambique, the potential of DSM soil survey methods to rapidly produce land suitability maps for a large area with acceptable accuracy was evaluated. Conditioned Latin hypercube sampling (cLHS) was introduced to determine field observation positions. SoLIM was used to run an inference with soil terrain rules derived from conceptual soil distribution patterns. A restriction of the expert knowledge based approach was found in that only six soil map units (SMU’s) could be determined per HA. The map achieved an overall accuracy of 80%. Land suitability maps were created based on the soil class map. In the Kruger National Park a soil map was used to create and extrapolate 2-dimensional conceptual hydrological response models (CHRM’s) to a 3-dimensional landscape. This is a very good example on how value could be added to a soil map. An error matrix convincingly identified problem areas in the map where future work could focus to improve the soil map. The current data indicates that at least 28 soil observations are necessary to create a soil map to an acceptable standard. When minimum observation criteria are met, observation density is irrelevant. The cLHS method to pre-determine observation positions improved the usability of observations. Although more research is needed to accurately determine the minimum observation criteria, an observation strategy is suggested. A 15 step protocol is produced with which it was shown that soil surveyors could produce a variety of maps in diverse situations. The protocol relies on the expert knowledge of the soil surveyor, combined with field observations. It has the advantages that fewer observations are necessary, map accuracy assessment is possible, problem areas are identified and under certain conditions unsurveyed areas can also be mapped. On the down side, there is a limitation of six SMU’s per HA. Further research needs to be done to determine the minimum criteria for soil observations, and soil distribution relationships between soil and remotely sensed covariates.
Afrikaans: Ten spyte van ʼn groeiende aanvraag vir ruimtelike grond inligting, is tradisionele metodes van grondopname te tydrowend en duur om in die behoefte te voorsien. Digitale grond kartering (DSM) metodes kan daardie leemte vul. Internasionaal beweeg DSM van die navorsings tot die produksie fase. Omdat grond-landskap interaksies en beskikbaarheid van data varieer tussen plekke, is plaaslike DSM navorsing nodig om die gebruik van DSM prakties uitvoerbaar te maak. Hierdie navorsing poog om ʼn werkende DSM protokol op te stel, wat gebruik kan word vir die kartering van groot land oppervlaktes in suidelike Afrika. Die protokol moet grondopnemers tegemoet kom, deurdat dit maklik genoeg moet wees om te volg, maar terselfdertyd toelaat dat produkte geskik vir die industrie geskep word. Om die skakel met die industrie te behou, is besluit om ʼn gevallestudie benadering te volg. Vier gevallestudies is in opeenvolging gedoen en die protokol opgegradeer na elke gevallestudie. Die gevallestudies hanteer ʼn verskeidenheid van uitdagings wat deur grondopnemers in die gesig gestaar word. In die eerste gevallestudie is ʼn basis protokol opgestel. Twee landtipes naby Madadeni was ontbind in ʼn reeks grondkaarte. Met elke kaart is meer inligting gebruik tydens die skep van die kaart. Vir Kaart 1 is slegs die landtipe inventaris en terrein analise ingespan. Met Kaart 2 is die kennis van die landtipe opnemer vir die gebied getap tydens ʼn verkenningsbesoek aan die studiegebied. Veld werk en ʼn vereenvoudigde grond assosiasie legende het ʼn toename in akkuraatheid vir Kaarte 3 en 4 veroorsaak. Dertig en 60 % van die observasiepunte is onderskeidelik gebruik as kwekingsdata vir Kaarte 3 en 4. Die akkuraatheid van die kaarte het toegeneem wanneer meer inligting benut is. Kaart 1 het ʼn akkuraatheid van 35% bereik, terwyl Kaart 4 ʼn geloofwaardige 67% akkuraatheid bereik het. Beginsels wat tydens die projek ontluik het, is dat veldwerk krities is tot DSM, meer inligting insette verbeter die uitsette en dat die vereenvoudiging van die kaartlegende die kaart se akkuraatheid verbeter. ʼn Onrealistiese eis vir ʼn grondopname van 37 000 ha binne agt dae deur twee grondopnemers in ʼn gebied met beperkte beweging a.g.v. ʼn landmyngevaar het die geleentheid geskep om die grondlandskap inferensie model (SoLIM) vir gebruik as DSM gereedskap te toets. Die gevallestudie het afgespeel in die Tete Provinsie, Mosambiek. Verdeling van die gebied in kleiner areas waar unieke grondverspreidings reëls geld (homogene gebiede) is tydens die gevallestudie ingestel. ʼn Vrye opname is geloods langs die beskikbare paaie van die gebied. Die finale grondkaart vir ʼn gebied van 15 000 ha beskik oor ʼn akkuraatheid van 69%. Die beginsel dat ontoeganklike gebiede gekarteer kan word, mits hulle in dieselfde homogene gebied as ʼn gebied waar ʼn opname wel kon plaasvind lê, is benut. Naby Namarroi, Mosambiek, is die potensiaal van DSM grondopname metodes om vinnig landgeskiktheidskaarte op te stel vir ʼn groot area getakseer. “Conditioned Latin hypercube sampling (cLHS)” is ingestel as metode om veld observasiepunte te bepaal. SoLIM is gebruik om ʼn inferensie met grond-terrein reëls afgelei vanaf ʼn konseptuele grond verspreidings model oor die hele gebied te dryf. ʼn Beperking op die deskundige kennis gebaseerde benadering is raakgeloop. Slegs ses grondkaarteenhede kan bepaal word per homogene gebied. Die kaart het ʼn algehele akkuraatheid van 80% behaal. Grondgeskiktheidskaarte is geskep gebaseer op die grond klas kaart. In die Kruger Nasionale Park is ʼn grondkaart gebruik om 2-dimensionele konseptuele hidrologiese reaksie modelle (CHRM) te skep en te ekstrapoleer na ʼn 3-dimensionele CHRM landskap. Hierdie is ʼn baie goeie voorbeeld van hoe waarde tot ʼn grondkaart gevoeg kan word. Satellietbeelde is gebruik tydens die skep van die kaart. ʼn Foutmatriks het doeltreffend probleemareas in die kaart uitgewys, waarop toekomstige verbeteringswerk op die kaart kan fokus. Die huidige data dui aan dat ten minste 28 observasies per homogene area nodig is om ʼn aanvaarbare grondkaart te skep. Wanneer die minimum observasie maatstaf vervul word, is observasie digtheid irrelevant. Die cLHS metode om observasie posisies te bepaal het die bruikbaarheid van observasies verhoog. Meer navorsing is nodig om die minimum maatstawwe vir observasies te bepaal. ʼn Vyftien-stap protokol waarmee bewys is dat grondopnemers ʼn verskeidenheid grondkaarte in diverse omstandighede kan skep is gelewer. Die protokol maak staat op die deskundige kennis van die grondopnemer, tesame met veld observasies. Die protokol het die voordele dat minder observasies nodig is, kaart akkuraatheid bepaal word, probleem areas uitgewys word en onder sekere toestande kan onbereikbare areas ook gekarteer word. Ongelukkig is daar ʼn beperking van slegs ses grondkaarteenhede per homogene gebied. Verdere navorsing is nodig om minimum maatstawwe vir grondobservasies en grondverspreidingsverhoudings met afstandswaarneming kovariate te bepaal.
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Keywords
Thesis (Ph.D. (Soil, Crop and Climate Sciences))--University of the Free State, 2013, Soil surveys -- South Africa, Terrain analysis, SoLIM, Remote sensing, Soil functions, Soil survey, Land type, Hydropedology, Inference systems, Expert knowledge, DEM, Conditioned Latin hypercube sampling
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