# Tag Info

21

The idea with hexagons is to reduce sampling bias from edge effects of the grid shape, which is related to high perimeter:area ratios. A circle is the lowest ratio, but cannot form a continuous grid, and hexagons are the closest shape to a circle that can still form a grid. Also, if you are working over a larger area, a square grid will suffer more from ...

18

One of the benefits, that I've seen when doing wildlife or habitat modelling especially, is that hexagons allow patterns in the data (ex, edge of a field or any other patch) to be seen more easily than what squares would of offered. Think of a soccer ball too, though not always hexagons, those geometric shapes fit to a curved surface quite nicely. In your ...

14

You need to know something about the meaning, method of acquisition, and processing of the elevation measurements, because slope calculations are fairly sensitive to resolution. You will get lower average slopes, typically, with a coarser resolution or when cell values are cell average elevations rather than spot elevations. In particular, if your grid has ...

14

Actually it's not all that situation dependent and is all about statistical error. Any time you resample to a higher resolution, you are introducing false accuracy. Consider a set of data measured in feet at whole numbers only. Any given point may be +/- 0.5 feet from its actual location. If you resample to the nearest tenth, you are now saying any given ...

12

The general approach for performing this kind of disaggregation is through dasymetric mapping, which uses ancillary data to inform the spatial distribution of a phenomenon, and is often used for population analyses, such as this one in San Francisco. This paper provides good background on the technique, and if you're working in ArcGIS, scripts have been ...

12

From someone who is part of the "GE Generation" Atmospheric Data When you are visualizing atmospheric features, it is important to see a vertical profile of the atmosphere. Sub-surface data When working sub-surface (bore holes, or earthquakes as shown below). In the image below, magnitude is mapped to pin size, but depth is mapped (inverted) to ...

12

chawkins I believe what you are looking for is called the World Port Index, the dataset is produced and maintained? by the NGA and can be found here http://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_pageLabel=msi_portal_page_62&pubCode=0015 The data is stored in an access DB the site also has a shape file which I've not looked at it yet but should ...

11

Spatial Statistics is probably classic example here. Also Spatial Data Analysis offers solid overview Statistical Methods for Spatial Data Analysis, Geospatial Analysis - a comprehensive guide and Geographic Information Analysis will give you nice overview as well. Another, more practical oriented way to go would be to look at R. Have a look at CRAN ...

10

For your specific question, you should be able to use point.in.polygon (docs) or overlay (vignette) to perform the analysis. A spatial predicate language isn't trivial to implement, and all the examples you mentioned boil down to ports or wrappers of JTS at some point. There is a pre-alpha release of rgeos, a recent Google SOC entry which provides access ...

10

I would suggest a raster approach with one raster layer for each criterion: bus quality (distance from pixel center to the nearest bus stop) shopping quality (distance to the nearest shop) flood danger (rasterize polygon layer, with danger grade attribute from 1 to 4) protection area (rasterize polygon layer) Then you can combine and weigh the layers ...

10

The link provided by @Mapperz mentioned that some of the potential sites "were considered at a MSL Project and MSL Landing Site Steering Committee meeting in Dec. 2009 that emphasized discussion of the science merit of the sites as well as landing site safety based on initial evaluation of thermal inertia, slopes, and other first order safety parameters." ...

10

The hexagon is the most complex regular polygon that can fill a plane (without gaps or overlap). I can see two advantages: It is closer to a circle than the square in terms of shape, so you suffer less from orientation bias (lower anisotropy with hexagons) and it is more compact (lower shape index: perimeter²/area). It therefore provides more accurate ...

9

I think both your points sum up the A to the Q quite well. Some examples where its useful: Flood Analysis Viewshed Analysis (including shadow analysis) Sub-terrain analysis (Geology, oil exploration) I think a key problem with 3D is the 'Google Earth Generation' (I just made that up), think that displaying everything in 3D is a good idea. This kind of ...

9

Since you have quite a lot of parcels that you want to test you should look for a software that uses spatial indexes for this type of calculations. And also be sure to use a method in that software that really uses the indexes. If you import your data into PostGIS you should use the ST_Dwithin function. Many people use buffer in combination with ...

9

You might want to give the Exploratory Spatio-Temporal Analysis Toolkit a spin and see how it works for your data. Developed by the GeoVISTA Center in close cooperation with users at the National Cancer Institute, the ESTAT toolkit provides user-friendly, open-source software designed to support exploratory geographic visualization. While ...

9

Yesterday I had no time to create it in details... See my solution in 4 steps: CREATE OR REPLACE VIEW bd_segment AS SELECT ST_PointN(geom, generate_series(1, ST_NPoints(geom)-1)) AS sp, ST_PointN(geom, generate_series(2, ST_NPoints(geom) )) AS ep FROM -- extract the individual linestrings (SELECT (ST_Dump(ST_Boundary(the_geom))...

8

Environmental investigations are inherently three dimensional. As a simple illustration of what some environmental data might look like, here is an image from a simple 3D GIS I created (using VRML) in the '90s. The box-like structures are buildings in an office park. The multi-colored "straws" display subsurface geologic readings taken every one to five ...

8

In ArcMap, If you look up Search in Polygon in ArcMap, you'll find this: "Select By Location to select those features from the points layer that intersect the polygon layer" That's found by "Selection>Select by Location" on the menu bar and it will ask for a Target layer then a Source layer and a selection method. So the Target layer is your point layer, ...

8

a and b are alias table names to the same table. This is effectively a T1 CROSS JOIN T2 in DB-speak. This allows a self-join to say "how close one part is to another" in a single table. SELECT a.hgt AS a_hgt, b.hgt AS b_hgt, ST_Distance(a.the_geom, b.the_geom) AS distance_between_a_and_b FROM public."TestArea" AS a, public."TestArea" AS b WHERE ...

8

Given a list of point locations (preferably in projected coordinates, so that distances are easy to compute), this problem can be solved with five simpler operations: Compute point-point distances. For each point i, i = 1, 2, ..., identify the indexes of those points at distances less than the buffer radius (such as 1500). Restrict those indexes to be i ...

8

This likely requires some scripting in any GIS platform. The most efficient method (asymptotically) is a vertical line sweep: it requires sorting the edges by their minimum y-coordinates and then processing the edges from bottom (minimum y) to top (maximum y), for a O(e * log(e)) algorithm when e edges are involved. The procedure, although simple, is ...

8

This is known as swept-path analysis. It is one of those calculations that seems initially to be relatively trivial but soon becomes obvious that there is a lot more to it because it is not just the tightness of the turn that is important. Some of the other things to consider include: Length of the vehicle and point of articulation. Turning circle of the ...

8

For a problem this small the slopes are easily computed with a simple raster calculation. Given that the years are consecutive, let's name the rasters [y.1], [y.2], [y.3], [y.4], and [y.5] in temporal order. The slope grid is (2/10) * ([y.5] - [y.1]) + (1/10) * ([y.4] - [y.2]) For other than five rasters--but still assuming they represent consecutive ...

8

I believe the AreaOnAreaOverlayer is the transformer that performs the equivalent of an ArcGIS Union. Performs an area-on-area overlay so that all input areas are intersected against each other and resultant area features are created and output. The resultant areas have all the attributes of all the original features in which they are contained.

7

An alternative that is well known in some circles but seems not to be known at all within GIS is Multi-attribute Value Theory. This is a theoretically well-grounded way to establish accurate scoring methods involving two or more characteristics (attributes). It proceeds by systematically considering trade-offs among attributes. With suitability problems, ...

7

Perform a hydrologic analysis on your data. Taking your first step of water bodies as a raster, you can then use that as a sinks raster. I'll specify the rest of this analysis in terms of GRASS as you mentioned that's the GIS system you're using: Set up r.watershed (documentation) with the elevation data layer you'd like to analyze and the sinks raster you ...

7

1 - There are some studies with this software: http://nlp.uned.es/MLQA06/papers/ferres.pdf Despite it's more related to internet searches, it could provide some guidance on how to translate human language to computer language. Googling 'GeoTALP-Q' also provides more articles on the subject. 2-GeoDjango provides an API for spatial queries, it's a translation ...

7

I can only think of 3 types of spatial query, ignoring any attribute or hash based queries. Spatial queries based on geometry, and are used to find the relationships between vector features. SQL spatial queries are really just an API low level alogorithms such as Bentley-Ottmann - used in OpenLayers to check if two lines intersect. As Kirk mentioned ...

7

Your surmises are correct. Checking for symmetry is an excellent idea: (Gaussian) curvature is an intrinsic property of a surface. Thus, rotating a grid should not change it. However, rotations introduce discretization error--except rotations by multiples of 90 degrees. Therefore, any such rotation should preserve the curvature. We can understand what's ...

7

These activities should all be easily done in QGIS, point by point as per your question: Symbols can be given radii based on a field. In the Symbology properties tab, select 'advanced', 'size scale field' and then the field with the radius (the CRS of the project will need to be in metres). The score can be stored in an attribute field; "Join attributes by ...

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