[GIS] Opening .shx files

shapefileSpecification

I want to see the contents of an .shx file to see how the index information is stored in these files so that I can get some help in writing my own shapefiles.

How can I open and use the .shx files?

Best Answer

The .shx file is binary, so you need a hex editor to look inside.

The format is described in http://en.wikipedia.org/wiki/Shapefile:

The shapefile index contains the same 100-byte header as the .shp file, followed by any number of 8-byte fixed-length records which consist of the following two fields:
Bytes   Type    Endianness  Usage
0–3     int32   big     Record offset (in 16-bit words)
4–7     int32   big     Record length (in 16-bit words)

Using this index, it is possible to seek backwards in the shapefile by, first, seeking backwards in the shape index (which is possible because it uses fixed-length records), then reading the record offset, and using that offset to seek to the correct position in the .shp file. It is also possible to seek forwards an arbitrary number of records using the same method.

You will find further links at the bottom of the wiki page.

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[GIS] Are ESRI Shapefiles and KML files two different ways of representing data

If you do not have a GIS Package yet, QGis is an excellent and free Application that will let you load up all your files and overlay them on top of each other so you can see exactly what is in them.

KML and Shapfiles could contain the exact same data, however KML is much more suited to displaying time based track information, whereas shapefiles are more suited to displaying Geometries, like boundaries, areas, roads, etc.

[GIS] How to read ArcGIS files in R

As handy as R is for so many tasks, it is important to remember that 1) R is not a GIS and 2) quality mapping is downright difficult compared to creating maps with QGIS or ArcGIS. The following example borrows heavily from two R-bloggers blogs (blog 1 and blog 2). Here, I simply mapped a polygon shapefile using Google Satellite Imagery as a basemap.

enter image description here

require(rgdal)
require(ggplot2)
require(rgeos)
require(ggmap)
require(RColorBrewer)

# Read shapefile using OGR
shp = "C:/temp/circles.shp"
myshp = readOGR(shp, layer = basename(strsplit(shp, "\\.")[[1]])[1]) # This is a fancy way of being lazy, so I do not need to type the layer name in

# Convert to lat long
myshp_proj = spTransform(myshp, CRS("+proj=longlat +datum=WGS84"))

# Find polygon centroid (This centers the map)
centroid = gCentroid(myshp_proj)

# Get the Google basemap
mapImageData1 = get_map(location = c(lon = centroid$x, lat = centroid$y),
                    color = "color",
                    source = "google",
                    maptype = "satellite",
                    zoom = 13)

# Convert shapefile to format ggmap can work with
polys = fortify(myshp_proj)

# Define the color scheme for mapping shp
colors = brewer.pal(9, "OrRd")

# create the final map
ggmap(mapImageData1) +
    geom_polygon(aes(x = long, y = lat, group = group),
    data = polys,
    color = colors[9],
    fill = colors[6],
    alpha = 0.5) +
    labs(x = "Longitude", y = "Latitude")

Best Answer

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[GIS] Are ESRI Shapefiles and KML files two different ways of representing data

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